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SOME EXPERIMENTS IN PIEZD ELECTRICITY
WITH ROCHELLE SALT CRYSTALS

THESIS run m: DEGREE M
MASTER or SCIENCE

HORACE V" CRANDALL
L983

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SOME mums IN P1120 ELEITRICITY WITH ROCHELLE SALT CRYSTALS

Thesis eubnltted to the faculty of Michigan State

College of Agriculture and Applied Science

by

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Horace VS Grandall

Candidate for degree of neuter of Science

June 1933

CONTENTS

Acknowledgements

Introduction

History of Piezo Electricity
General Treatment

Piezo Electric Rochelle Salt

EXperimental and Constructional Data
Growth of Crystal
Sectioning the Crystal
Mounting the Crystal
The Phonogrsph Pick-up
The Crystal Loudspeaker

The Crystal Microphone

Xfray and Crystal Structure
Conclusion

Bibliography

94940

20

21

37

41

48

49

52

57

59

ACKNOWLEDGESENTS

To Dr. Ewing and Mr. Eek of the Chemistry Department for their
timely suggestions and generous help in my first successful attempt

to grow Rochelle salt crystals for this exPeriment.

To Professor Miller and George Chapman of the Physics Department
for their very capable assistance in the construction of the two adjust-
able thermo-regulators which were made entirely in their labortory and

Bhope

To Professor Snoe,also of the rhysics DepartmentIfor his coopera-
tion in the investigation made of the crystals by x-rays and the inter-

pretation of the resulting Laue patterns.

To Mr. Osborne of the Electrical Engineering Department for his
sound advice on the many questions arising from this problem and for

his generous help with the illustrations presented in this thesis.

And last.but by no means least,to frofessor hurray,also of the
Electrical Engineering Department,for his never failing interest in
the work and his most valuable assistance in locating many articles of

reference on the subject.

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INTRODUCTION

The selection of the tepic of piezo electricity in Rochelle salt
was not made because of its research possibilities. Neither was it be-
cause the phenomena are something new.for piezo electricity has been
known for well over a hundered years. Rather,was it selected to bring
to the attention of many the possibilities which lie in the everyday
application of these phenomena.

In this field much credit is due the Brush Deve10pment Company of
Cleveland Ohio,to whose work many references will be made in this paper.
It was this company that first successfully commercialized the production
of Rochelle salt crystals for such applications and who are now making
splendid advancements in the field.

To the many scientific investigators of the phenomena is due much
credit for making possible this new field of industry. They are mention-
ed with full credit in the historical presentation to follow.

May those who read this paper find in it,in some measure,the satis-

faction and pleasure which has been the authofs upon the successful

completion of this work.

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HISTORY OF PITCZO ELECTRICITY

G ENE} RAL TR’TI A'leEI-i T

The Curies are accredited with having discovered the phenomena of
piezo electricity in 1880 - 1883. However,these phenomena have been known
for well cVer a century. In 1820 - 1833 A.C. Becquerel made many experi-
ments with piezo electricity and tested a large number of substances,crys-
talline and otherwise,for these effects. Even as far back as 1703 we find
records of a Dutch jeweller heating tourmaline crystals on embers and find-
ing that they attracted small bodies to them upon cooling. This discovery,
the first recorded,was,as many such findings are,purely accidental. And
yet it would be interesting to know just what the man was actually search-
ing for at the time.

The discovery of the Dutchman,coming as it did before the existence
of two kinds of electricity was known,had no significance attached to it
and went unpublished except for the mere mention,anonymously.in a German
publication,a small pamphlet published in 1707 under the title of
"Curious Speculations in Sleepless Nights". Notice was given to the extent
that from then on tourmaline became known as the electric stone or as
the Ceylon magnet.

From 1880 to 1883 the Curies,later of Radium fame,made many experi-
ments with pieso electricity. They demonstrated that tension and compress-
ion have the same effect as hesting and cooling respectively. A squeezed
crystal of tourmaline behaves electrically as a cooling crystal,and the

expanding crystal resembles a heated one. To distinguish the various

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phenomena the electrification due to pressure was designated as piezo
electricity; piezo from the Greek piezcfin meaning to press or pressure.
The generation of electricity by heating was termed pyro electricity;
pyro,also,from the Greek word pur meaning fire. It is generally believed,
however,that piezo and pyro phenomena are due to the same or at least

to similar reactions in nature.

The Curios published in 1880 the results of their experiments with
quartz. These included a quantitative measurement of the amount of elec-
tricity generated by unit pressure along various axes of the substance.

In 1881 Lippman predicted from mathematical considerations that ifv
quarts sas subjected to an electric field,deformation of the quartz
would result. This sas tested by the Curios and found to be the case.
They pointed out that from the las of conservation of energy any piezo
electric substance.which acts as a generator of electricity in response
to mechanical motion,will act conversely upon application of electric
potential. The first practical application of this converse principle
see made by the Curios in the form of an electrostatic voltmeter.

The honor of being the first to suggest acoustic applications of
piezo electric substances goes to Roentgen of X-ray fame. This came in
1890 as a result of his study of torque as produced in several quarts
cylinders when subjected to electrical charges.

In their investigations the Curios made determinations of the piezo
electric constant‘the charge per unit stress)for many crystalline sub-

stances,smong which Rochelle salt possessed by far the greatest.

Little was actually done in this direction until the Great War came,
with its submarine warfare. Then many investigators took up the task of
finding some means of detecting these death dealing monsters from the
depths in time to thwart their plans. The application of the piezo elec-
tric properties of these crystals,coupled with the newly invented three
element vacuum tube as an amplifier of the small potentials from the crys-
tals.wss the contribution of the brilliant French investigator,Lengevin.
With the crystal-tube combination at both input and output ends on the
circuit,he was able to detect the presence of subm'rines by the super-
sonic vibrations given off by the boat as it progressed through the water.

Although he was interested mainly in super-audible frequencies,he

demonstrated the possibilities of these crystals in the audible range.

PIEZC ELECTRIC ROCHELLE SALT

Some of the first investigations of piezo electricity.and its poss-
ible applications in Rochelle salt crystals was made by the Western Elec-
tric Company about 1919. A.M. Nicolson,writing on the subject of "The
Pieso Electric Effect in the Composite Rochelle Salt Crystal” in the
Proceedings of the American Institute of Electrical Engineers for October
19l9,reached very definate conclusions about the possibilities of commerdial
application of such crystals. The crystals of his investigations were
characterised by their lack of homogeneity and the inclusion of mother
liquor. Experience has shown that,due to deterioration and lack of uniform
properties in such crystals,on1y clear and homogeneous crystals can be
used with anything near success.

Let us consider some of the early investigators of these crystals,
beginning with the work of Joseph Valasek.of the University of Minnesota,
and his associates. This work was published as early as 1921 in the
Physical Review of that year. Various other articles have since appeared
in the same publication giving further advancements and findings of these
men. Let us consider first Mr. Valasek's article as published in the 1922
Physical Review in which he says in effect:

We shall begin by reviewing seme of the general facts concerning"
Rochelle salt. This substance is a double tartrate of sodium and potas-

sium,having a chemical formula Na K C4 H4 06 4 320. It has a molecular

weight of 282.19 and a specific gravity of 1.77. It crystalizes in the

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ortho-rhombic system,showing sphenodial hemehedrism. The crystalline
form appears as trimetric prisms. It is Optically active both as a cry-

stal and in solution. The structural formula appears as:

0-0 0 H
We
Na-é-O H
é-O 0 H

This form has the usual type of symmetry occuring in optically active
carbon compounds. The crystal is brittle and soluble in water,thus per-
mitting cutting and polishing with water. Since it cracks easily due to
too rapid or unequal temperature changes,it must be handled carefully.
It cannot be heated above 53 degrees centigrade as a crysta1,since at
this temperature it transforms into water and single tartrates of sodium
and potassium,with the absorption of heat.

With this brief and general knowledge of Rochelle salt let us see
what has been done with it during the_recent. investigation of its prop-
erties. Assuming we have successfully grown the crysta1.a not altogether
easy task,the preparation of it for such tests as we may see fit to make
is Quite well described by Mr. Frank Isley in the Physical Review of 1924.
A block of crystal is cut as shown in figure 1. Since it has been known
for some time that the maximum activity is in the plane of the (b) axis
perpendicular to the (a) axis we will select this plane for our examina-
tion. We will then employ the usual method of cutting with a wet string

and grinding by means of emery powder and water. The finishing polish is

 

 

 

 

 

 

 

 

 

I Figure l
c

given by water and a ground glass plate. After shaping,the crystal is
coated with asphaltum cement,which is made from asphaltum gum and bensine.
This cement in no way alters the crystalographic structure of the crystal
and reduces to a minimum the change in vapor tension. A piece of tin foil
or lead foil is also cemented to each side of the crystal to which is
attached a lead-in wire for applying or removing the potential. With such
a prepared crystal and a delicate weighting instrument the crystal is sub-
jected to pressure between two brass plates. An electrometer can be used
to measure the potential generated. A vacuum inclosed chamber with heat-
ing and cooling facilities is necessary for temperature determinations.
With the apparatushsseindicatedflr. Isley obtained data for the set of

curves as shown in figures 2,3,and~4.

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1924 page 571
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Physical Review Vol.24
Figure 4 1924 page 573

Following a very similar procedure Mr. John G. Frayne made another
series of investigations using,instead of direct potential polarisation,
a.high frequency alternating potential. He termed the resulting response the
‘Reversible Inductivity of Rochelle Salt Crystals Under High Frequency
Fields" and published it in the Physical Review of 1922. Mr Frayne defines
his reversible inductivity as the limit of delta (D) over delta (E) as
delta (E) approaches sero,where (D) is the induction and (E) the electric
field in the dielectric. The plate used for his examination by Mr. Frayne
was the conventional cut in the (b) plane perpendicular to the (a) plane
to obtain maximum activity. It was mounted between the two plates of a
condenser as the dielectric. Capacity determinations were made by the
resonance method using a field oscillating at two mega-cycles per second.

He found the reversible inductivity (Kr) was about one tenth the dielectric

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constant of the crystal as measured by the direct potential methods.
Variations of (K?) with temperature was obtained between -80° C.

and 50° 0. This relation is shown in figure 5.

 

 

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MATURE - Degrees Centigrade
J.G.FRAYNE
Figure 5 Physical Review Vol.21

1923 page 355

It will be noted that the~ value of (Kr) increases with temperature direct-
ly from -80° to -20°. Then from -20° to 4° it decreases. From 4° to 23°
11'. again increases and from 23° tothe melting point of the crystal it
decreases. The concave portion of the curve between -20° and 23° coarse-
Ponds to the range of greatest pieso electric activity. The dielectric

constant. as measured by the direct potential method varies in the opposite

13

manner between -20° and 23°,being a maximum at about 4°.as shown in
figure 6,which was taken from an article by Joseph Valasek in the Physical

Review for 1922.

 

 

 

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TEMPERATURE - Degrees Centigrade
JOSEPH VALASEK
Figure 6 Physical Review Vol.19

1922 page 488

The value of (Kr) was measured for different states of polarisation
(of the crystal. The high frequency potential was superimposed on a direct
field. Different samples of crystals used showed permanent polarization
effects. The crystals became conducting and the capacity infinite for
innaller values of field strength in one direction than in the other.
(harves showing the variation of (Kr) with polarised fields are shown in

fIlgure 7.

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A - Natural field of crystal , A‘ - Natural field of cry-
with applied field in one stal with applied

rx direction along crystal. field in opposite

a.
53 direction along

crystal.

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ELECTRIC FIELD - volts/cm.
J.G.FRAYNE
Figure 7 Physical Review Vol.21
1923 page 357

The fact that Rochelle salt crystals experience fatigue has been
observed by various experimenters. W.G. Cady in his report to the National
Research Council in May 1918 described these effects as not of a permanent
nature but existing for many hours. Valaeek finds that this may account
for the capricious manner in which the pieso electric modulus varies in
response to a number of conditions such as temperature,humidity.and pre-
vious history with regards to electrical and mechanical treatment.

In the equation for charge (q) on a crystal of length (1),breadth (b),
and thichnees (d).for a total force (F) on the end of the crystal we

have: .
q: ~314r(1/ze)
Here ~314,the pieso electric modulus.variee from practically zero to

12

4.0 x 10 esu/dyne. He finds that humidity increases the response directly

and that temperature varies the activity as shown in figure 8.

15

 

 

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JPONSE — strain/stress — cm./dyne

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-50 -40 -2o 0 26 4o 60

 

TEMPERATURE - Degrees Centigrade
Joseph Valasek
Physical Review Vol.19
1922 page 485

The actual fatigue and recovery of a crystal is shown in figure 9 with

Figure 8

accompanying explanations.

Fatigue and recovery of Rochelle salt condenser after
charging for 24 hpurs with plus 100 volts then apply-
ing a negative 100 volts to the plates.

 

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Josepfi Valasek

 

 

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1924 page 564
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Figure 9

16

The variation of the pieso electric constant,that is the charge re-
sulting per unit applied force,is shown in figure 10,shich curve is due
to Mr. Valasek and was taken from sun article by him as published in the

Physical Review for 1922.

 

 

 

 

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I 1600‘-
E:
E3
‘5 1200”
O
[D
H
33’
E3
cg 800“
a:
[\‘1
FT]. '8
h: 400 .L 138 x 10
___ ' 28e3 X 1‘
0 g t t 5 1 i t
-60 -40 -20 0 20 40 60
TEMPERATURE - Degrees Centigrade
Figure 10 Joseph Valasek page 641

. Physical Review Vol.20
Many attempts at explanation of the phenomena of pieso electricity

have been made by various authorities. Perhaps the best presented to
date is that of Joseph Valasek as it appeared in the Physical Review for
1924. Mr. Valasek states: ”It is interesting to note that the high dielec-

tric constant of Rochelle Salt can be attributed to the water of crystal-

l7

isation without requiring greater displacement than the distances between
molecules.“

“The maximum polarization obtainable is less than 15,000 e.s.u./cm.3
There are four molecules of water for every molecule of Rochelle salt._
The crystal lattice of each molecule of water may be regarded as equivalent
to three ions.two hydrogen and one oxygen. Suppose the restoring force/

unit displacement/unit charge is the same. Then:

P ' 16 N E R

max

Where: Pmax - the maximum polarisation

E the electronic charge

R

the displacement

N the number of molecules of salt / cm.3

which is equal to 3.76 x 1021.'

11 cm. However the value of (R)

”This gives (R) a value of‘5 x 10-
'from a consideration of the number of molecules present (N) is only

(R) 3 3 x 10'? cm. as the distance between molecules. Thus it is possible
for the water of crystalisation to furnish the ions for internal conduction
and yet not move out of the molecule"

In support of this theory Mr. Valasek offers the experimental results
shown in figure ll,as taken from the Physical Review for 1922. In all
fairness,hosever,ur. Valasek states: "The decrease in maxima and also
their displacement is in the same direction as,and may be due entirely to.

the effect of the different dielectric properties of the crystal.and of

the dehydrated layer. In other words the presence of a layer of inactive

18

dielectric of relatively low specific inductive capacity will diminish the
charge on the plates due to polarization of the active layer,and thus de-
crease the piezo electric response. It will also diminish the effective
field across the active layer making it necessary to increase the potential
difference between plates to produce the same field across the inner layeq

thus shifting the position of the maximum activities."

10 "

— Ci
_ 4
U!

10%

'FLIJCT

D“
H
.L.’

2 o <\\\‘

o - . 1 . 1 c . - - .

-16 -120 -so -40 0 40 so 120 150

 

APPLIED POTENTIAL - volts

Chart showing the effect of drying upon Rochelle salt crystal.

A - response before drying.

B - response after drying in chamber with onsfor 1 day.
c_ n n a n n n “'3dQYIe
D- . n ' n a n :0 10'0an

J as eph .Valas ek
Physical Review Vol.1?

Figure 11 ' 1922 page 482

19

Despite Mr. Valasek's frank presentation of the possibility of the
results of figure 11 being due to some other causes than the actual loss
of pieso electric properties,by loss of the water of crystalization,the
fact still remains that the loss of this water does decrease the activity,
whereas the increase of water present,in the form of increased humidity,
gives rise to greater activity. This in itself is a fair proof that the
water of crystalization is,in a great measure,responsible for the phenom-

ena of pieso electricity,as observed in its maximum activity in Rochelle

salt e

WWENTAL AND CONSTRUCTIONAL DATA

21

GROWTH OF CRYSTAL

In order to carry on any experiment with Rochelle salt crystals/
some source of crystal supply must be at hand. Since the whole procedure
was one of experimentation it was decided to do some work on the growing
of crystals at home,so to speak,uith what facilities there were at hand.
Being quite unfamiliar with the chemistry of crystal grosth,the author
was stubs: to know just how to begin. All articles on the subject dealt
very briefly.or not at a11,upon this very essential phase of the work.
After exhausting all available articles without finding anything of prac-
tical value,a review of local possibilities was made. Upon the suggestion
of some of the students majoring in chemistry,ur. Eck,of the Chemistry
Department of the College,was interviewed on the subject. His suggestions
were of a quite general nature. He stated that at no time had he found it
necessary to produce crystals as large as those required for this work.
His suggestion was that an attempt be made to grow then from a slightly
supersaturated solution of the salt by the process of slow evaporation,or
'by'a temperature gradient method with a more concentrated solution. As
for the latter method no data was available as to just what the temperature
Egradient should be in degrees per hour or day,nor as to how saturated the
solution should be.

D A review of some of the early literature on the subject,however,led
t<> the discovery of some previously overlooked suggestions on this latter
Inetdiod of growth in an article by A.“. Nicolson in the Transactions of

the A.I.E.E. for 1919. This article stated.in effec't,that Rochelle salt

22

crystals are grown from perfect nuclei possessing definate form. The nuclei
or seed crystals are immersed in a saturated solution of salt under identi-
cal conditions of temperature. A density of 1.33 at 50° C. can be used con-
veniently and the seed crysta1,previously warmed to the same temperature as
the liquid,should be inserted between the temperature of 38°Cand 35° C.
Continuing,Mr. Nicolson ssys,"The crystals may be grown by application of
temperature gradients to a saturated solution of the salt.or by concentra-
tion brought about by slow evaporation. The former,producing a specific
type under the conditions of rapid cooling,is the method preferred. The
crystals may be grown in the mother liquor by suspension from a clean
thread.by flotation on mercury,or by being laid on a glass plate,the latter
two methods being preferred."

With this material as a basis,the production of some Rochelle salt

crystals of suitable nature was attempted. A solution was made of the rec-
ommended density. Having no seed crystals at first,the solution was set
aside and left to cool over night. The next day a plentiful supply of cry-
stals was found on the bottom of the jar. Some of the more perfect of these
were removed with care and set aside for future use. The solution was then
reheated to the proper temperature,about 50° C. ,and allowed to cool to
37° C. At this point one of the small' crystals,which had been warmed to the
sexes temperature in an improvised oven made from a hinged cover wooden box
with an electrical heating element of the common variety in it,was placed
in 'the solution and the whole allowed to cool as rapidly as it would in the
air' of the room,until the next day. The inspection of the jar the next day,

however,was very disappointingnor not only had the seed crystal grown

23

but also numerous others had formed and grown so that the entire bottom
of the jar was filled with crystals,all intersecting one another and no
perfect ones to be found.

The same general procedure was followed on the next trials with the
slight variations of incidentals usually made. The temperature of setting
the crystal was varied,as was the density of the solution. The temperstuse
gradients were decreased by placing the whole jar in the oven and allowigg
it to cool more slowly than before. Much care was exercised to prevent
jarring and thus forming extra crystals from the supersaturated solution.
The results were always the same with the exception of the number of
crystals forming at onesetting. The smallest number formed at onetime was
three. These were arranged symmetrically about the bottom of the jar,the
(c) axes of the crystals forming the spokes of a wheel 120°apart and all
intersecting at the center. Of course even this was unsatisfactory for
our needs.

The next variation of method was to try the second suggestion of Mr.
Nicolson and grow the crystals on mercury placed in the bottom of the jar.
This,however.met with a similar fate and no single crystals were grotn by
this method.

Next the growth of the crystals,as a complete type,wss attempted by
suspension from.a fine wire. This was tied about the crystal and then
suspended from a small rod placed across the top of the jar.as shown in
the photOgraph of figure 12. It often happened,however,that the crystal
would dissolve a little when first placed in the solution and would there-

by become small enough to fall from the retaining loop of wire. It then

24

 

FIGURE 13

25

became necessary to reset the jar with a new seed crystal. Here was a
point to be improved upon and several other methods of fastening the seed
crystal to the wire were tried. The best and most satisfactory of these
was that of melting a small hole in the end of the crystal with a hot pin,
inserting the suspension wire,and allowing the crystal to cool and harden
about it. By so doing.and inserting the wire sufficiently deep into the
crysta1,it became impossible for the crystal to fall from the wire until
it was practically all dissolved.a thing which rarely happened. This method
of suspension also made a smaller hole in the crystal,when sectioned.than
did the first method. This was a great help later in being able to use
more of the crystal,since large crystals seemed very difficult to obtain.

This suspension method of growth proved the best of all methods tried,
as it kept the seed crystal away from the growth which invariably took
place at the bottom of the jar,the thickness of which can be seen in fig-
ure 12:‘ I

It was found necessary.in some cases,to place a piece of copper plate
just under the surface of the solution,suspended by wires from the top of
the jar. This was to catch the little crystals which are continually form-
ing on the surface of the solution and growing until they become large
enough to sink,for in so doing they often strike and adhere to the seed
crystal,producing a parasite which grows with it and prevents formation of
perfect specimens.

After having a fair degree of success with this method of growth,and
having accumulated some twelve or fifteen crystals,from one and one half
to two and one half inches long and an inch to two inches thncJ[.,the next

problem was some manner of cutting. Several methods were tried.which will

26

be discussed in detail later on. Upon the first cutting it was found that
what had appeared to be a clouded portion of the crystal was really a
smallpocket including some of the mother solution. Naturally this spoiled
the whole crysta1,since the pocket had a peculiar faculty of always forming
at right angles to the plane of greatest piezo electric activity,namely
the (b) plane,thus separating the section into two or more parts,which
were then too small to be of use. Besides being small,these sections were
filled with tiny cracks and crevices due to the unequal growth of the
crystal. Since all crystals grown in this manner were more or less clouded
or cracked it was necessary to find some other method of growth which
could be better controlled.

Luckily,at this point,the attention of the author was called to an
article in the Journal of the American Chemical Society for 1919.entitled
9‘ Method of Growing Large Perfect Crystals from Solutionz by Raw. Moore.
In this article Mr. Moore put forth a method which he had used with great
success for growing Rochelle salt crystals of large size and perfect clear-
ness and symmetry. This method was.in effect,as follows: Prepare a satur-
ated solution of Rochelle salt at a temperature of between 3596and 40° C.
Then,after removing the excess salt from solution.heat to about 7° or 8°
above this temperature and filter,being careful that the temperature at
no time falls below 4° or 5° above saturation temperature. The seed cry-
stal should then be inserted and the jar placed in a large container of
‘wster at .5° above the saturation temperature of the solution. The water
:is allowed to cool to saturation tempersture,and the thermo-regulator

sand heater arrangement then maintainsit at this temperature until read-

'F‘

CIr

\

-‘J

sev-

27

justed. The cooling gradient recommended was .1° the first day, .2° the
second day,and from .30 to .6° each succeeding day. If more than one cry-
stal is grown at a time in the same jar,this gradient may be increased.
Mr. Moore furnished with his article a chart,of specific gravity of the
saturated solution of Rochelle salt plotted to temperature,as a guide in
preparing the solution for a certain temperature range. This chart is re-

produced here,figure 13,with all credit given Mr.Moore for its origin.

Saturation Chart for Rochelle Salt

1.42
1.40

1.38

1.35 ~
1.34
1.32
1.30

1.28

IC GRAVITY (IDTDROEL‘STER)

1.26

1.24

 

 

1.22 Ir 3 C 3 v ‘ A t I - s A .
e 10 12 14 15 1s 20 22 24 25 22 30 32 at 36 as 46—

TEMPERATURE - Degrees Centigrade RxW.MOORE
Figure 13 Jour.Am.Chem.Soc.

:It .111 be noted that for the range indicated as best by Mr. httrttnfiiefiylglg
k>etween 35° and 40° C. ,the specific gravity of the saturated solution is

taetween 1.37 and 1.42 as compared with the value of 1.33 at 50° as given

28

by Mr. Nicolson. It has been the experience of the author that the values
as given by Hr. Moore work much better than those suggested by Mr. Nicolson.
Having no facilities for the control of temperature to an accuracy of
.5° available in the Engineering Department,the assistance of the Chemistry
Department was again solicited. In this department were found so called
'mercury regulators' which would maintain a constant temperature to .005°,
provided the bath was kept in constant motion. The jarring caused by this
agitation of the bath,however,would have proved fatal to the growth of
single perfect crystals. As the necessary temperature regulation was only
.50 it was thought probable that this instrument would maintain the desired
regulation by means of convection currents alone,withoutagitation.providing
the heating coil was placed in the bottom of the tank. Through the generous
cooperation of Dr. Ewing and Fr. Eek the apparatus was assembled,as shown

in figure l4,and a trial run was made.

 

 

 

7
'J

 

 

 

 

 

 

 

 

 

 

 

 

 

 

V m

 

 

 

 

 

 

 

 

 

 

 

Figure 14 1 ' ‘
A - Glass Aquarium 3 - Relay
' B - Mercury Regulator F - Dry Cell
C - Thermometer G - Ring Stands
D - Beaker containing solution H - Capper strips supporting I

H
I

and crystals Sheet Rock plate holding
15 Electric Heating Element beakere

29

For a setting of 37° the temperature range was from 36.50 to 37.5°. This
was thought to be satisfactory. The crystals were then set after the method
described by Mr. Moore,and develOpments eagerly awaited.

The crystals grew very well for the first few degrees drop. Then the
limit of adjustment of this regulator was reached. In attempting to read-
just the regulator the capillary tube at the side was broken off. This
appeared quite a catastrophe in view of the fact that it was borrowed equip-
ment. It must be repaired: Having had a little experience in glass blowing
at the Physics Department,the author decided to attempt the repair of the
broken regulator. The attempt was so successful that it was decided to make
a regulator with a greater range of adjustment. This was done very nicely
with the generous assistance of Professor killer of the Physics Department.
The finished regulator is shown at the right in figure 15. At this point a
brief description of the regulator,and its more sturdy companion on the left
of the same figure,is not out of place.

The bulb at the bottom was made from a 50 cc. round bottom flask. To
it was attached a short piece of r/z inch glass tubing by drawing down the
neck of the flask. The L tube on the right was then inserted. It was made
by sealing a four inch piece of 1/16 inch capillary tubing into an eight
inch length of 1/2 inch tubing leaving about one and one half inches at one
-end and about three and one half inches at the other. The long and was bent
at right angles just below the end of the capillary tube and sealed ’into
time neck cf the flask just above the bulb. This completed the glass work.

The regulator was now thoroughly cleaned,first with gasoline to remove
all grease.then with the conventional glass cleaning solution of caustic

Potash,and finally rinsed with distilled water and dried in an air blast.

30

 

w ,-
' ’.’."."’I//r"n.1n

 

 

 

FIGURE 15

31

Then the clean mercury was placed in the bulb filling it to a height of
about one inch in the tube proper. The work was then ready for the adjust-
ment device.

It was desirable that the regulator be adjustable over a rather wide
range. To accomgrlish this an adjustable plunger was inserted in the main
tube to regulate the height of the mercury in the side tube at a given
temperature. This plunger was a leather washer,held between two brass wash—
ers.and attached to the end of a small threaded rod by a little bolt insert-
ed in a tapped hole in the end of the rod. The long threaded rod extended
out the top of the glass tube through a cap made from a small capper cylin-
der into which was soldered a brass nut of the same thread as the rod. The
cap was then slipped over the end of the glass tube and held firmly in
place by a heavy wax. A cast off radio dial was then placed at the end of
the threaded rod to afford an easy means of turning it and of marking its
setting.

A similar procedure was carried out for the side tube except that the
plunger extended only into the one and one half inch space at the top ofths
capillary. A small iron wire had been soldered on to the threaded rod and
‘this continued on about three inches into the capillary tube. The threaded
rod was attached as before to the top of the tube and a smaller radio knob
served for'itSadjustment. The purpose of this fine wire in the capillary
‘wss that of completing the circuit through the regulator and as a micro-
meter adjustment for the plunger.

The regulator was designed to work in conjunction with a relay in the

Power circuit. The method of operation is similar to that of an ordinary

32

thermometer with the exception of the electrical circuits involved. Since
mercury has a relatively high coefficient of expansion for temperature
change,is a liquid at ordinary temperatures.and is an electrical conductor,
it is ideal for our purpose. The large quantity of mercury contained in the
bulb gives ample expansion for small temperature changes. This expansion
may take place in two directions,up the main tube or up the capillary tube.
By means of the adjustable plunger in the main tube the amount of expansion
in that direction can be controlled at will. Thus by turning down the plun-
ger the mercury is forced up the capillary tube . When it comes in contact
with the small iron wire,projecting down into the tube,there is a completed
electrical circuit between the cap of the main tube and that of the capil-
lary tube. Thus the caps of the two tubes become the terminals of the reg-
ulator which is connected in series with the control relay and 'a power
supply. At this temperature and plunger setting,the relay circuit is closed
,th'ereb'g opening the power or heating circuit. Leaving the plunger set
at this point,the mercury will remain in contact with the iron wire until
'the surrounding media has cOoled sufficiently to reduce the temperature
and volume of the mercury to a point where the thread of mercury in the
capillary tube breaks contact with the wire. The electrical circuit is
‘then broken and the relay opens,closing the heating circuit. When enough
Jhest has been supplied to the system to raise the temperature sufficiently
to expand the mercury up the capillary tube to contact with the wire,the
'whole cycle is repeated and the temperature oscillates between two points.
The value of this swing is determined by three factors,the amount of mer-

cury in the bulb,the size of the capillary tube,and the rapidity with which

33

the heat is distributed through the water in the tank,or in other words
whether the water is agitated ot not.

After the regulator was completed it was roughly calibrated by placing
it in a dish of water with a thermometer and heating the water. After con-
tact had been made between the caps,the plunger was raised one turn.and a
note was made of the number of degrees rise in temperature required to
expand the mercury until contact was again made. This was found to be about
5° per turn.and,as the dial was etched,almost any value could be selected
by turning the dial only a few divisions. The micrometer adjustment of the
side tube could be added to this and very fine regulation maintained.

Being made of glass.and containing so much mercury,the regulator was
quite sensitive to jar. It was so delicate,in fact,that several were broken
before one was completed and put in operation. To minimise the chances of
the regulator getting a damaging bump while in operation,it was placed in a
perforated metal container and packed with excelsior. Thie,of course,length-
ened the time required for the temperature change in the water to reach the
regulator and thus increased the swing of the temperature between its limits.

To overcome the difficulties involved in the first regulator a second
one was constructed. This was more sturdyly made throughout,having all possi-
ble glass parts replaced with similar ones of iron piping,as shown at the
left of figure 15. It was built along the same pattern as the original one
where ever possible. The only glass used in the construction was the cap-
illary tube.which served two purposes. It magnified the effect of the ex-
pansion in the mercury and served as an insulator between the terminals of

the regulator.

34

It was found necessary to place a rubber washer at the end of the plunger,
despite the fact that the threaded rod fitted tightly into the inside of
the pipe.which had been machined to a smooth surface with only six threads
left at the top. Most of the other variations are visable in the photograph.
The capillary tube was waxed into the tube with the same heavy wax as was
used in the first case. This regulator replaced the original for a short
time. For some reason it was difficult to keep a temperature setting with
it,presumably due to the leakage of mercury past the washer in the tube.
Because of this it was eventually set aside in favor of its more delicate
but reliable predecessor.which was used throughout the rest of the experiment.
Having now a regulator of our own.so to speak,it was decided to re-
turn to the greater convenience of the Engineering Department to continue
the experiments. Here an arrangement of apparatus,similar to that used at
the Chemistry Department,was made. A large fifteen gallon crock was sub-
stituted for the glass aquarium and a small 10 volt toy transformer and
an A.C. relay for the batteries and D.C. relay previously used. The general
idea of the arrangement Ian be obtained from the photograph of figure 16.
The crock was insulated with two layers of Balsum Wool on the sides and
bottom. The jars of solution were set in the crock on a shelf of sheet
rock board which was supported on two hangers hooked over the tap of the
crock. The heating element was the wire from an ordinary 500 watt heater
which had been unwound and coiled back and forth at the bottom of the crock.
The regulator was placed in its metal container and rested on the shelf
along with the jars of solution. A thermometer was hung in the crock to

keep record of the temperature at all times.

35

 

 

 

 

FIGURE 16

36

With this apparatus,and a plentiful supply of Rochelle salt,some very
fine crystals were grown. They were perfect in symmetry and as clear as
glass although somewhat smaller than the ones that had been grown before.

Of course.accidents will happen,and one of these cost the loss of a
large jar of solution. This happened as the waste crystals.which had form-
ed at the bottom of the jar,were being remelted in preparation for a new
setting. This was being done by placing the jar in a pan of cold water and
gradually warming it by admitting live steam into the water. It seems that
the crystals expand a great deal more than glass for a given change in
temperature,and,in so doing,invaribly break the container. As this happened
when no one was around,the entire jar of solution ran out into the pan of
water and was lost. Experience was a very good teacher.but an expensive
cne.and after that the crystals were broken out of the bottom of the jar

with a pick before trying to redissolve them.

37

SECTIONING THE CRYSTAL

Having grown several crystals of the proper size and clearness,some
means of cutting or sectioning these crystals must be found,since the com-
plete crystal is of little value piezo electrically. It was decided to try
several methods of doingthis and to use the first grown crystals,since they
were more or less clouded and therefore of little value even if successfully
cut.The method which had been previously used by experimenters in the field
was that of a wet string which was moved back and forth across the crystal.
thus dissolving a strip as wide as the string through the crystal. This was
a tedious process and not applicable to quantity production. The Brush De-
velopment Company of Cleveland Ohio had successfully used machine methods
of sectioning these crystals quite similar to those used in woodworking.
and with very critical cutting speeds. Exactly what the procedure was,and
the value of the critical speed,was naturally omitted in their article in
the November 1931 issue of the Proceedings of the Institute of Radio Eng-
ineers.

The first woodworking machine tried was the band saw. A jig was used
to hold the crystal and this rested on the table of the saw. After cutting
half way through the crystal it suddenly cracked the remaining distance
and fell apart. As was to be expected,however.the break was not in the de-
sired direction and the crystal was spoiled. Several other trials were made,
holding the crystal in different positione,but all with the same result.
The crystal always broke apart due to the vibration of the saw or from in-

ability to hold the crystal properly. Since the speed of the saw could not

38

be changed.or the size of the blade varied,it was decided to try a small
hand saw of the coping variety,using a very fine blade. The same trouble
was still present. It was impossible to hold the crystal solidly without
ekerting sufficient pressure to crack it. Holding it loosly permitted it
to slip and this invaribly kinked the saw and snapped out a piece of cry-
stal. From the adverse results of these trials it was apparent that some
other means of cutting must be found.

The next attempt was to cut the crystal with a wire. The wire soon
became covered with the salt and stuck in the crystal. To remedy this the
wire was heated by passing an electric current through it. This dissolved
the adhering salt and cut the crystal but in so doing it cracked it.due
to the unequal expansion near the wire. This'eliminated the hot wire as
a cutting tool.

Returning again to the idea of a saw for cutting the crystal.a high
speed paper disc was tried. This disc was about four inches in diameter
and made from the paper cover of a laboratory folder. It was attached
directly to the shaft of a motor designed to drive a refacing wheel for
lathe use.and ran 17,500 r.p.m. The motor was mounted in the tool rest
of the lathe and the crystal was held firmly between two wooden jaws
clamped between the jaws of the chuck. The disc was advanced to the crystal
by the lathe screw to keep it in perfect alignement. The cutting was sat-
.isfactory,from that point only,but the heat developed by the speed of the
disc was as disastrous as that from the wire in the previous trial.

Having exhausted the available supply of tools for cutting purposes,

the last resort was the old method of using a wet string. This was slow

39

and tedious,but at least it gave satisfactory results in the way of clear
and uncracked sections. A photograph of the improvised cutter is shown in
figure 17.

The crystal was held between two soft pine blocks which slid in a
groove cut in the supporting piece. These blocks were firmly held together
by adjustable thumbscrews through angle irons attached to the base. They
were soft enough to prevent breaking the crystal yet rigid enough to hold
it securely. The thread,which was used to cut the crystal,was moved back
and forth across it by hand. Guides were placed in front and behind the
crystal to keep the thread in the correct cutting position. The thread
was kept wet by passing it through a dish of water as shown in the photo-
graph.

After properly orienting and securing the crystal in place the thread
was passed over the pulley,through the water,and across the crystal. Attach-
ed to the pulley end of the string was a small weight which hung over the
edge of the table. The string was pulled across the crystal,lifting the
weight,which always kept it taut. Upon releasing the string,the weight
pulled it back across the crystal again.and all was ready for the next cut.
Omcprecaution should be taken particularly,and that is to be sure that the
water used in cutting is the same temperature as the crystal. This is to
prevent cracking due to too rapid temperature changes in the crystal if it
A is exposed to cold water.

With this apparatus,about five minutes is required to cut a crystal.
The usual cut is to halve the crystal along the (b) axis. The remaining

portion of the crystalnear the outside is either cut away or ground off.

40

The crystal is then polished by using powdered carborundum and water on

a smooth surface,finished by ground glass and water. This left a thin slab
of crystal from the center of the piece,perpendicular to the (a) axis and
in the (b) plane,as shown in figure 18. This is the most active plate that
canbe cut from the crystal unless the one cut at 45° to this one is consid-

ered,as shown in the dotted lines of figure 18.

 

 

 

 

 

 

 

 

 

 

Figure 18

41

MOUNTING THE CRYSTAL

After the crystal had been sectioned,some form of mounting must be
considered. As before mentioned there are two possible arrangements of
the crystal for best results. If the crystal is as first cut,shown in the
solid lines of figure 18,the expansion.upon application of potential,is
in the (b - c) plane parallel to the (b) axis. If the crystal is held
firmly at (1 - m) the (j - k) portion will move back and forth in synchro-
nism with the applied voltage. Obviously this is one way of mounting the
crystal. It may be clamped firmly at one end and the desired movement taken
from the other end,as described above. This might be classed as arrangement
"A",for simplicity. Another arrangement of almost equal simplicity is that
of a section cut from the crystal at 45° to the (b) and (c) axes in the
plane of these axes,as shown in the dotted lines of figure 18. Such a plate
will be acted upon in compression and extension where the first mentioned
plate.responds to shear. Let us denote the latter plate as arrangement "B”.
Obviously either of these methods would produce a piezo electric bar,or
p1ate,which,when energized from either electrical or mechanical sources.
would function as the simplest form of electro-acoustical instrument it
is possible to conceive.

It is found,however.that in this case,as in many cases,the simplest
arrangement is not the best one for perfect results. As has been mentioned
in the articles by Mr. Valasek,included in the first section of this paper,
Rochelle salt crystals functioning piezo electrically,experience fatigue

and hysteresis loss.depending to a greater or less extent on several factors

42

 

 

 

 

 

 

FIGURE 17

FIGURE 19

 

43

already discussed. To overcome this difficulty several remedies have been
tried. It was found that if the entire plate was clamped and prevented
from moving.to any great extent,upon application of potentia1,the hyster-
sis loss and saturation of the crystal disappeared. It would have been
impossible to use a crystal so clamped,but,by cementing another similarly
cut crystal to the first,in such a manner that the forces are always op-
posing each other,a similar result was obtained. At the same time great
magnification of movement results,but in a direction at right angles to
that experienced before.

Compare,if you will,such an arrangement to that of the bi-metallic
thermostat. With two sections cut as in A,and cemented together in opposi-
tion,an element results which twists upon application of potential,or
creates potential upon being twisted,the action being reciprocal. If out
as in B the movement is one of-bending,very similar indeed to the bimetallic
strip upon application of a temperature gradient. In either case the move-
ment is proportional to the impressed voltage,or vice versu.,and no hyster-
sis is present. The frequency of the applied force is also immaterial up
to at least 500,000 cycles per second.

Owing to the small size of the crystals obtainable,all mountings
were made after arrangement A. This gave an element sensitive in torsion,
which must be used accordingly. Figure 19 shows the three stages of pre-
paration of the crystals for use in these experiments. The steel scale,

calibrated in inches,gives an idea of the size of the crystals.

44

THE PHONOGRAPH PICK-UP

The actual making up of one of the crystals into a usable unit took
the form of a phonograph pick-up,for playing ordinary phonograph records
through the radio or other electrical amplifiers. It was made from a piece
of extremely clear crystal,which had_unfortunately broken in two after
having been uded for some time in experiments with loudspeakers. The two
pieces were cleaned and shellaced together again in opposition with a
piece of lead foil between them. This center foil served as one terminal
of the instrument. The other terminal was the two outer surfaces which
had been foiled in a similar manner. To one corner of the crystal assembly
was cemented a grooved piece of pressed fiber,which had been drilled and_
tapped to accomodate the phonograph needle and holding screw. The unit was
then mounted in a flat circular head which was also made from a piece of
pressed fiber. The needle box,as the receptacle for holding the needle is
known.projected from the case through a hole in the side. The cover,which
was machined to fit tightly into the head,was of the proper thichness to
hold the crystal unit snugly in place and prevent motion,when once bolted
together. This cover also had a cavity of the proper size cut in one side
to accomodate the needle box,which,of course.must be free from all solid
contact with the head,except through the crystal itself. Included in the
head was a small .001 uf. condenser connected in parallel with the crystal
unit to bipass the scratch of the needle on the record. Smaller details
of the arrangement can be gathered from the photograph,figure 20.which is

a close-up view of the pick-up head.

4s

 

 

 

 

 

FIGURE

20

 

 

 

 

 

FIGURE

21

46

The complete unit was mounted on a pivoted arm which was free to
rotate on its base,ss_shown in figure 21. The head was so constructed that
it could be used to play either the conventional lateral cut record or the
new type vertical cut recordings. All that is necessary.to change from one
type to the other.is to loosen the retaining screw in the end of the arm,
remove the head,turn it a quarter turn and reinsert it in the arm. The ver-
tical position of the head,as shown in figure 21,13 for playing lateral cut
records. The horizontal mounting plays the vertical type. This seemingly
reversed nomenclature results from the necessity of picking up the vibrations
from the record in such a manner as to cause the proper torsion in the crys—
,tal unit,thus producing the piezo electric potential for the amplifier
excitation.

In comparison with the magnetic type pick-up we find the crystal unit
a very capable competitor. It is inherently simpler and under proper man-
ufacturing facilities probably less expensive. No method of damping,as
commonly employed,is necessary,since the resonant frequency of the crystal
is well outside the audible range. Its response to high frequencies is
better than the magnetic unit and here,perhaps,trouble is encountered.due
to the high frequency scratch from the record,unless excellent records and
proper needles are used. Impedance match must of course be made for best
results. The impedance of the unit is such that it may be inserted directly
in the grid circuit of the first amplifier either with or without a grid
leak across it. Transformer coupling may be used but trnasformers tend to
suppess the low frequencies and should be avoided whenever possible. The
unit itself has the characteristic of a leaky condenser of about .01 uf.

capacity the leakage ,however.being small.

The actual frequency response of the unit could not be determined
due to lack of calibrated sound instruments in the laboratory. However,
the response,as determined by the ear,the final criterian in any case,

was entirely satisfactory to all.

47

48

THE CRYSTAL LOUDSPEAK ER

Many experiments were preformed in an endeavor to produce a loud-
speaker of suitable Volume and fidelity. It must be admitted here that
such aspirations were not entirely fulfilled. Due to the difficulty in
producing crystals larger than about one and three quarter inches long
by an inch or so thich,it was impossible to get sufficient motion to
successfully drive the eight inch cone. The faithfulness of reproduction
was good at low sound levels,but,in attempting to increase the volume,
all quality was lost. Since the crystal itself could not be overloaded
without great loss of fidelity,a mechanical lever was tried in an attempt
to increase the movement. Any mechanical arrangement is inefficient,and
as much was lost here as was gained from the lever ratio. That such a
speaker canebonstructed which will favorbly compare with the best dynamic
units can be shown from the success of the Brush Development Company of
Cleveland 0hio,who have produced such units for the past year or two.
They used a crystal which measured two and three quarter inches by three
and one half inches,so it is little wonder that the results obtained from

the comparitively tiny one used in this experiment were so unsatisfactory.

49

THE CRYSTAL MICROPHONE

Since the crystals responded very well at low sound levels it was
decided to try to construct a microphone from some of the crystals which
had proved too small for a speaker. The crystal unit itself was made from
the small crystal shown in figure 19. The unit was assembled for sensitiv-
ity in torque with terminals brought out as in the pick-up. It was mounted
between two fiber pads,as shown in the photograph of figure 22.and securely
held to the heavy pressed fiber base by two small bolts. The diaphram for
the unit was made from a light manilla paper cover of the familiar labora-
tory report,in the absense of any better material. It was securely clamped
between two rings made from the same fiber board as the base. The bottom
ring served as a spacer,for the support of the diaphram at the proper level
above the crysta1,and provided ample space for mounting. A small threaded
rod was cemented to one corner of the crystal at the center of the base.
This served as a means of coupling the crystal to the diaphram,which was
attached to the rod by two small nuts.

The ring was attached to the base by three bolts,which fitted into
tapped holes in the ring,but did not project through it. Thus a neat unit
was assembled. The view of the assembled microphone is shown in figure 23.

The microphone,while less sensitive than the conventional double
button carbon type.is much more sensitive than the condenser microphone.
It will furnish good volume with an amplifier consisting of a type '27
driver and two '470s in push pull,although another stage would reduce

the noise level considerably. The quality,as determined by the only avail-

50

 

 

 

 

 

 

FIGURE 22

 

 

 

 

0'.

\J

 

 

FIGURE 23

51

able means,the ear,was excellent. Absolute calibration,by the Brush Devel-
opment Company,of one of their microphones,showed a straight line from 50
to 5000 cycles with a gradual drop below 50 and a similar rise above $000.
This was accomplished by addition of parallel resistance. However,the cry-
stal unit alone varied less than 8 decibels over the entire range. Almost
any impedance characteristic desired can be obtained by either parallel or
series arrangement of several of the crystal units. In this case.of course,
the units themselves act as the diaphram.with a corresponding decrease in

sensitivity and increase in fidelity.

52

X - RAY AND CRYSTAL STRUCTURE

During the course of these experiments the author had occasion to
become interested in some work which was being done in the x-ray field
at the Physics Department,by Professor Snow. Having acquired some new
equipment.a special room was being set aside for x-ray work alone,some-
thing previously impossible. In trying to work out a course in this field,
Professor Snow invited several of the advanced students to participate in
some experiments he proposed to perform. Being desirous of keeping up with
the times as much as possible in all lines of endeavor the author at once
availed himself of the opportunity. In the course of these eXperiments the
investigation of crystal structure by X-ray analysis was mentioned and dis-
cussed. Having at hand a General Electric Xeray Spectrographic Outfit it
was decided to actually preform the experiment and thereby get a little
information 'first hand' gas to speak.

It immediately occured to,the author to try some of the Rochelle
salt crystals in an attempt to find out more about their actual construc-
tion and,possibly,the reason for their piezo electric action. To this end
many trials were made. Between poor technique of handling the apparatus
and films,and some bad accidents to these films,the first few trials were
not very successful. However,by keeping at it,and gradually improving the
methods of procedure.some very good Laue patterns were obtained._Three of
these are shown in the accompanying photographs. They are respectively:
{Figure 24,with the crystal perpendicular to the (a) axis;figure 25.with

the crystal perpendicular to the (b) axis;and figure 26,with the crystal

FIGURE 24

 

FIGURE 25

 

FIGU RE 26

 

56

'perpendicular to the (c) axis. The similarity of figures 24 and 25 indicate
similarity of strusture in these planes,while the totally different figure
26 shows an extremely different arrangement of atoms here. Time did not
permit the actual working out of the crystal structure,even if such were
possible from these photographs,since such a problem is a thesis by itself.
The lack of symmetry in figures 24 and 25 is due to inaccuracies in grind-
ing the crystals and in mounting them in the‘machine. Each succeeding trial
yielded much information as to better methods of procedure in taking the'.
photographs,but this was costly to those taken in that trial. One example
of this is the extremely light spot in the center of the plate due to the
undeflected beam of x-rays. It was found later that this could be partially,
if not,wholly,eliminated by placing a small piece of lead in front of the
photographic plate at this point. This,of course,would result in a much
clearer plate near the center by the elimination of scattering due to the
emulsion on the plate. '

The actual information yielded by these Laue patterns is not great.
It is,howeVer,interesting to note that X-ray furnishes a means of actually
looking into the atomic structure of things. The use of these patterns,in
conjunction with those made from mono-chromatic radiations in spectrograph-
glic work,provide very excellect methods of obtaining this priceless in-
formation about the internal arrangement of materials. Had time and facili-
ties permitted,nothing could have pleased the author more than to have
investigated the crystals more thoroughly by this means. As it is,the photo-
graphs serve only as a very crude example of what can be done with X-rays.
along the line of crystal structure,and to this end add a little color to
the eXperiments with Rochelle salt crystals and their piezo electric prop-

'erties.

57

CONCLUSION

In conclusion let us summarize the advantages posessed by Rochelle
salt electro-acoustical apparatus over the older magnetic types,and the
existing electro-static types,as well as cite some possibilities in
application to other fields.

First,the use\ of Rochelle salt devices offers outstanding cheapness
and simplicity coupled with a flexibility of design very difficult to find
anywhere else. The life is extremely long.and failure from fatigue is un-
known,under ordinary working oonditions,with the sectionalized unit as
herein described. Such apparatus as has been discussed,and perhaps many
other applications,exhibit real individualism in that they,unlike all
magnetic types,require no permanent magnets or heavy currents for field
excitation. Unlike the carbon microphone,no batteries are required for
power,and no polarizing bias is necessary as with electrostatic instruments.
In all these cases Rochelle salt is indeed unique in that it carries its
own field excitation.

When connected to an input circuit there is ample potential generated
to swing the grid of almost any suitable tube sufficiently for complete
utilization of that tube. In output circuits the impedance match is direct
with most power tubes on the market today. For multi-unit installations
the low impedance necessary can readily be obtained by the proper arrange-
ment of units. Also,the force present in output units is indeed great,and
can be used to good advantage in driving the heavy cones necessary for

good reproduction at high sound levels.

58

The application of the piezo electric crystal in modern science is
indeed just beginning. Besides the speaker,microphone,and pick-up these
crystals have been successfully used in polarized relays,whsre potential
is present but where very little power is available. They also find appli-
cations in oscillographs and oscilloscopes. Another very tempting applica-
tion is in the matter of seismographs,for continuous automatic recording
of earth tremors,such as might be caused by an earthquake several thousand
miles away. It is apparent that such movements of the earth as would result
from such a cause would indeed be small. But,emall as they are,a device as
sensitive as Rochelle salt crystals should pick them up with very little
difficulty. That they were used successfully during the Great War to detect
submarines,by means of minute water vibrations,is proof of their sensitiv-
ity. And in the light of modern developments this sensitivity has been
increased many times.

There are undoubtedly scores of other applications,equally interest-
ing,which are eagerly awaiting recognition and deveIOpment. May they not
be long in coming,and they will,not if some of the gifted engineers,like
those at the Brush labortories,get to work on the problem. The savings
which are effected by such applications,both in labor and capital,should

be a great incentive for the immediate development of these devices.

59

x BIBLIOGRAPHY
Cady.'W.G. .
Piezo electric Terminology. Proceedings of the I.R.E. 1930
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Cady,‘W.G. A
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l6 : 521 — 35. April 1928

Frayne, J.G.
Reversible Inductivity of Rochelle Salt
Physical Review 1922 20 : 97 v ’ (530.5 P 578)
Physical Review 1923 21 : 348 ,

Isley, F.C.
The Relation Between the Mechanical and Piezo electrical Properties
of Rochelle Salt Crystals.
Physical review 1924 , 24 : 569 v (530.5 P 578)

Moore, R.W.
A Method of Growing Large Perfect Crystals from Solution.
Journal of the American Chemical Society 1919 41 : II : 1060
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Nicolson, A.M.
The Piezo electric lffect in Composite Rochelle Salt Crystals.
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Sawyer, C.B.
Rochelle Salt Crystals for Electrical Reproducers.
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Scott, E.K.
Piezo electricity in Rochelle Salt Crystals
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(Valasek, J.
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60

Piezo electricity and Its Applications
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Note: Parenthesis,( - ),indicate classification in the Dewey Decimal

system used in the Michigan State College Library.

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