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we c/CIRCJDateDtnpes-nu

THE PHOTO-IEECTRIG EFFECT
01‘
ITNILI DIVIDED CARBON

a. .n...
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AND

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Tho-1| for 13153130 of M. S.
Wesley E.‘"_‘I‘homu
1927

HISTORY OF PHOTO-ELECTRICITY

The first important observation of photo-electricity
was that of Hertz in 1887, when he found that a apart would peel
between two plates more easily if the negative plate wae illumi-
nated by the light of another spark. This effect led. Hallwack,
lleter, Gertel and others to emeriment with metal plate: and
they found that negatively charged plate: lost their charge Ihen
they were illuminated by a suitable source of light. In 1896 Sir
J. J. Thomson explained the nature of the conductivity inputted
to gaeee by various agents. especially x—raye and thereafter made
very mid progress in the investigation of photo—electricity and
condnction through gases. Ethan Sir J. J. Themeon and Lenard
observed that a metal plate in a mac would emit electrons if
illuminated by ultra—violet light. These electrons have a
velocity of 107'cm/uc.. while those of the cathode rays have
a velocity of 10m erg/sec“ shouting that the electrons observed
by Thomson and. Lenard were slower than electrons of the cathode

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Stoletow produced some evidence of photo-electricity
when he performed the following experiment. He had a solid plate
”A“. Figure I. and a perforated plate ”3" and a very high resist~
ance galvanometer of high sensitivity. He had a source of poten—
tial, the negative of which was connected through the galvanometer
to the plate "A” and the positive to the plate ”B“. He found that
when the plate “A” was illuminated the galvanometer indicated a
flow of electricity, but this was not so if the connections were
reversed. He also made a photo-electric cell which supplied a
current to the outside by its action. A solid plate "A”, Figure II.

was used and a perforated plate "B" and a quadrant electronter was

used in place of a galvsnometer. Stoletow found that if he allowed

.2.

ultra-violet light to pass through the Openings in the perforated
plate that current would flow. He used the sun as a source of
light. The conclusions of Hichs. Stoletow and Arrhenius were not
quite correct. 'mey believed that the current produced was due to
the contact difference of potential between the plates and the air
between the plates and diaelectric which trade the two plates the
same potential. This difference in potential must be considered,
but the main action is due to the light, which liberates negative
charges from the illuminated plate.

Very little was published until 1898 and then the articles
were scattered and not until about 1910 did photo—electricity become
popular among physicists. In 1905 H. S. Hower performed experiments
on “The Production of Ozone by a Photo—Electric Current". Ultra-
violet light was allowed to fall upon a polished platinum knob in an
oxygen atmosphere. The knob was charged to a negative 1500 volt s,
and the discharge current, from it to the earth wires Opposite the
knob, was measured by a galvanometer in series. The scene produced
was detected by its effect or. a piece of platinum foil. The tail
changed its position in the voltaic series when it absorbed ozone.
In 1908 I. F. Holman contributed data on "Fatigue and Recovery of
the Photo-Electric Current". The same year F. K. Richtmyer con—-
tributed data on "The Dapendence of the Photo-Electric Current on

Light“. In 1910 Millikan proved, to his own satisfaction, linstiens

equation for the “Emission of Electrons“. Then in 1911+ J. Robinson
investigated "The Velocities of the Electrons Emitted". He found
that for very thin films (less than 10-7 cm) the maximm velocity
of emission due to the emergent light was greater than that due to
incident light; for thicker films the reverse was found to be true.
since the 11311me energies of emission are independent of the illum—
ination. Partzah and Hallwacks concluded that the variation in
absorption of the two positions of the films would be no adequate
eqlanation of this observed change in velocity. Robinson also
determined distribution of velocity (tn-es. for four samples of
different thickness of films.

Since 1916 the field has had my contributions on photo-
electricity, but very little on finely divided carbon and for that
reason I am trying to offer some data concerning l'E‘inely Divided
Carbons”. In addition I am reporting some of my results on the

photo-electric activity of certain dyes.

PHOTO -ELECT RI C AOI‘ 101!

In photo—electricity we are dealing with electrons, but
because of their connection with this effect they are called Photo-
electrons: however. they differ in no way from other electrons. The
difference pertains to the method of liberating them, and yet even
here, there is a mind similarity. Ordinary electrons are liberated

from gaseous and liquid setter while photo—electrons are liberated

from solid nutter. mthfcrd advanced the theory that the atom
itself is a couple: system built up of a positive nucleons of
greet couplexity carrying a positive charge, which is surrounded
by a number of electrons. each carrying negative charges: the
atoms differing according to the element.

lith this in mind we seem to be able to classify electrons

as follows:

1. Fr” electrons to which the electrical conductivity of
metals is due.

9. Dispersional or Emission electrons which give rise to
the absoxyticn of light and are responsible for the
zeenan effect.

3. The valency electron which corresponds to the chemicel
bond.

It. The Photo-electrons which are separated from the atom
by the influence of ligit.

The second and third are identical and when an absolute classification
is made, probably all electrons are identical, but the task here is to
prove that the photo—electron is also a member of, sniidentical with.
the other three. ‘

It night be well to review some of the previous works on

this subject. The work of Pam and Pringsheim certainly suggests that
the electrons of the selective effect are connected with the chemical

binding of the atom. According to the theory of Star]: also. the

electrons of the photo-electric effect are the valency electrons.
Lenard distinguishes between the photo—electric electrons and the
emission electrons. and on this distinction bases his theory of
phosphreecence. rho excitation of a phosphorescent substance by
light consists in the separation of the photo-electrons. These

on their return to the atompset in vibration the emission electrons.
so giving rise to the luminescence. Erma theory 1. not well founded
as the initial velocity is independent of the intensity of the light.
Lewis and Lengmir have developed a very suggestive theory as to the
geometrical arrangement of the electrons in atoms. Aseundng with
Rutherford that the positive charge of the atom is concentrated in
an exceedingly small nucleons. it is supposed that the electrons

are distributed through a series of concentric mhericel shells:

the effective radii of the different sheels stone in the ratio
13233:“ and the effective surfaces are in the ratio 1:22:322“2.

In the mtherford-Bohr theory of atomic structure the atom resembles
a planetary system in which the planets are electrons. which circu-
late about the central positive charge or nucleons. The electrons
may be divided into a number of groups which. in the earlier pre-
sentation of the theory. were regarded as shells enclosing one
another. In his latest investigation (which will be discussed later)
Bohr regards a “shell“ as a closely bound group of electrons although
each individual electron can at times approach the nucleons. or recede

to an infinite distance from it. Ie say. however. distinguish between

o

the “peripheral“ and “central“ properties of the atoms Thus
the x-rey spectrum has its origin in the innermost orbits of
the atom. while the optical spectra are produced at the per-
iphery. she electrons which.are liberated gy_ordinary light
oo- from the W the “0%ng those refionsible
for its chemical behavior.

let us next ask how the electrons escape from the
surface of mtals or any other surface. under the influence
of light. Several theories nave oeen advanced to explain this
liberation. and difficulties heme been found in most all theories.
The corpuscular theory of ligt seem to offer a solution. whose
tiny particles stricking against the plate at a very high velocity
will disturb the atomic balance and electrons are emitted. Ins
difficulty encountered.here is that we have no well defined foun-
dation for the corpuscular theory. Another theory for this lib-
eration is the “explosion“ theory. Again we assume that the come
plex atomic system is unstable. The theory is that photo-electrons
being in an'unstable condition are completely thrown off balance
by the action of the short were lengths of light. If this is the
case the energy of the escaping electrons is derived from within
the system. thus the photo-electric effect would be sindler or
equivalent to an induced radio-activity. Ibis, however. cannot
be so because we have experimental evidence that radio-activity
is entirely independent of all external conditions. Another

theory is that the electron is liberated because of the velocity

given to it by the passage of a half-wave of light. The kinetic
energy of the emitted electron is derived from the energy of the
incident light. The difficulty here is, the mgnitude of the
initial velocity is too large for it to con from a half-light
vibration. Further the velocity of emission is independent of
the intensity of the incident light. we not: from experiments
that the photoeelectric effect is instantaneous. If the emitted
electrons obtained their velocity from a spreading wave—front
the photo—electric effect would not be instantaneous but soon--
tise would elapse before the action could be detected. this is
true because a spreading wave-front could not ispart all the
energy to the electron, all in a lump and cause instantaneous
action, but the energy would be inparted to the electronin
very smll incri-ents and the accumlation of these incrimsnts
would case the photo-electric effect. Again the resonant
vibration has been suggested as a possible explanatien of this
phenomenon. he electrons are set into vibration by incident
light and thus acquire sufficient energy to enable them to
escape. 11' light of the same frequency is incident upon the
electron so as to excite linear resonance vibration, the am-
plitude of the vibration will go on increasing until its mg-
nitude becomes sufficient to free the electron from the atom.
However, under these conditions its velocity would be were.

In order that the electron might leave the atom with finite

velocity it not obtain its energy from the last half of the

-s-

wave length or whole resonant period and this cannot be true as
is explained in the theory above.

Lenard assumed the existence of complicated conditions
of motion of the electron within the body and that the initial
velocity is not derived from the light energy but from the energy
of these movements already existing before illumination takes
place. In this case the resonant vibration only acts as a
liberator, the electrons being liberated when it reaches the
major axis of its orbit and well out to or beyond the limit of
attraction. This theory is very good hit again we are confronted
by the question, "Is it possible that all wave-lengths of light
will prothice resonant vibration", yet it is a well established fact
that all wave-lengths do produce photo-electric effects of varying
intensities.

Einstein in an endeavor to connect some of Plank's
results from radiation of black bodies or "h” (Plank's universal
constant of action) with photo-electricity brought out this theory.
The Kinetic energy of the liberated electrons is tuna. The equation
for the liberation of photo-electrons is hv - I? II M, where "h“ is
Plank's constant and is equal to h a 6.555 x 10’27 ergs/eec., “v"
the frequency and “P“ the work necessary to liberate the electron.
This liberating energ is the product of the potential difference
1 and the charge ”e“. At first this theory seemed to be without
foundation, but was later proved by Millikan to be correct, that

is he proved it to his own satisfaction. According to this we

new write the equation for the liberation of electrons by light
thus: hv - P ' i I72 3 1e. A slight modification of Lenard's
theory enables us to see that this equation.holds for that theory.
Let hv 8 energy of the electron within the atom; P I the energ
necessary to liberateothe electron: iflvz the energy of the liber-
ated.electron,‘and.!p the product of the difference of potentials
and the charge to give the electron the Kinetic energy of Q We.
on. mdified linstein's equation will read: 1. a give I! hv I r:
“I" in this case being derived from the light that has acted only
in the role of liberator. The energy of the free electron being
that derived from within the atom. Arriving at this sen con-
clusion from two independent sources we are Justified in beliefing

that the theory is correct.

-10-

Bohr's Theory of the Atom

Bohr had two very definite fundamental hypotheses, -
the first one is that for each atom or atomic system there
exists a number of definite states of nation, called “stationary
states” in inch the atom, or atomic vstem, can exist without
radiating energy. A finite change in the energy content of the
atom can take place only in a process in which the atom passes
comletely from one stationary state to another: the second
hypothesisdstates that if such a change takes place with the
emission or absorption of electro-nlgnetic light waves, these
waves will have a definite frequency, the magnitude of which
is determined by the change in the energy content of the atom.

If we denote the change in energy by'l' and the frequency by

I I we may write 1: fin : 9: 11:; where w is the Plant constant.
In consequence of the second hypothesis the emission as well as
the absorption of energy by the 'atom always takes place in quanta.
~ h - 6.555 x 10'27 nylon. Another statement of I is E 3 E1 - '2"
and is usually written A I.

If for the hydrgen atom we consider that the orbit of
the electron is circular with the misleus as the centre, and if
we consider the mass of the nucleus infinite compared with that
of the electron, the centrifugal force is P - Mfr, (l' '3 centri-

fugal force, a 3 mass, _l_ = angular velocity) and the force of

-11-

s
V A f . -
r I .
A . v c
. h k
. - ‘
.. ‘ '
.
w
. ’ at
t
.m- .— ' .
‘ - >45
‘ - ' —-
. .-
. !
~—

.
I
.
~ . .-
' -
a
w
-J
u a,
1 ,
.
. q ‘ -.
V
—
' e
.
‘
...
‘ v
' ‘" s

attraction between the electron and the mcleus is ca, because
the charge on the melons is positive (4-) and that of the
electron is negative (-) and they are equal, and “r“ is the
distance between the electron and the nucleus.

Since the force out and the force in must be equal
for a system in equilibrium «-
lfiiiar' 23. or rear} - .2 (1)
KJ. Came; I a “2: by definition (from classical theory)

.Yilr (180mg)

.°.K.E.=§M!2r2endrwidfi2r 7
The lunetic energy of the atom C M212. PJ. is the potential
energy or the work necessary to bring the electron from infinity
to the nucleus. This P.E. due to -e at a distance r from the
nucleus is :93 the work done being negative because the charge
on the election is negative. The total energy of the atom “I“,
or the work done, is LE. 0 P.E.
w=n+rn=%Mfier2-g:°; but "Ezra-3
thenI-gE-gE-Lefi r r (1:)

2r r 2r

If the orbit changes from radius "1" to radius “2" the change

in energ of the atom is i - 21

2132 2r1
but according to the quantum theory -
2.2..-93_=h't= (m)
are 2131'

where 't I the frequency of the emitted energy.

The quantum specification of Somerfeld is a foundation for.
this, which is fldq a nh

q ’- eny defining ordinate of the wstem

P 3 corresponding mnentum

n 3 a whole number

For hydrogen atoms the orbit being circular, we may express
the motion in polar co-ordinates: fir.

r is constant: 1 99¢; let q '6: P 3 Mira
gfl' dt
unread an.
0

2P1 H1132 I nh

were '2...
2pi

|i'he whole angular unnentum 15ng is an intrigal number of tines

.11...

2331
Then from (I) aft-3 9 e2 (1.)

r I! e2
M1232

substituting values of r2 and 12 in terms of equation (1.)

and r '3 nah2
1Tia-i e211 (IV)

Then from equation (11) eibstituting values of r; equation (IV)

will read - £212 a“ g
n

now from (III) if N1 and la are orbits -

2 h 2 h
hvt . gpi e I - gi e g
n h n2 he

.13..

O.

a- —.

 

-
h
N
a»-
M_‘.
e
.— «-_

“*‘fi—

 

Or Vt ‘g212.nn (fi-iz'...)
I . 12 u 1 "1 eee
' 5“ QfiJ<fi '2 )

1“2
v=R(l -l
“‘2
1“1

R 3 109732; v 1: wave number

.11}-

Sources of Light

During the very early experiments in photo-elec-
tricity the spark was the only artificial source of light.
This source however, gave a very limited supply of light
of the short wave lengths and as the experiments became
more intensive more adequate light sources were sought.
The spark naturally led to the arc as a source and it was
found that the arc gave a mach richer supply of light in
the short wave lengths. The one great difficulty encountered
by using the arc is the extremely high temperature present
resulting in uncertainty. This may be overcome by placing
a shield over the arc and insulating and ventilating it so
that an air current is created, the direction of the air
current being from the plate toward the arc and not vice-
versa.

Another exceptionally good source of light is the
quartz-mercury-arc. This is a simple mercury are enclosed
in a quarts tube. The quartz tube is essential as most
other substances such as glass would be opaque to wave
lengths shorter than 3500“ if this tube is preperly
constructed and the radiation of heat is adequate little

effect of tenperature will be realised. This source is

rich in short wave lengths and almost free from.the longer

wave lengths. In using the quarts tube the thickness is a

very important factor. Fused quarts 6 mm thick will trans-
mitt light of 1250 aM but the thicker the quarts the more

opaque it become to short wave lengths.

For our purpose the arc is used. It is shielded
snd.properly ventilated that the air currents are from.ths
plate to the arc snd.thus the temperature effect is reduced
to a minimm. Then we used the are as it is more flexible
and variations of light can be easily obtained if it should
be needed at any tine. Therefore for our purpose the carbon
arc, using 3/8 inch solid carbon rods. seem to be the most

practical.

-16-

Photo-electric Fatigue

The decreasing of the photo-electric effect of metal
plates and other substances after being exposed to light'for a
period of tine is called ”PHOTO-ELECTRId FATIGUE". This phenom-
enon has been known from the very beginning by experimnters.
Herts, Hallwacke. Hoor. Stoletow. Elster and Geitel all observed
that metal plates aged rapidly when exposed to light. Hallwacks
found that after ageing the plates. almost no photo—electric
effect was realized from a plate that was photo-electric before
exposure to light. lhat is the'cause of photo-electric fatigue?
1. Is there a chemical change? (Oxidation)
2. Does fatigue dopend upon the sise of the container?
3. Is there a roughening of the surface or a physical
change?
It. Is there an electrical change, the formation of a
double layer?
5. Is there a change in the surface due to a film of
gas, or in the occluded gas in the metal? '
One thing met be kept in mind, however, and that is the phenomenon
is not always the sane. each substana might and probably does vary

according to its conposition.

It is not likely that there is a chemical change
booms/e. Xreusler has proved that a sinc plate that had been
polished and kept in the dark for hours showed an activity
equal to a freshly polished plate. Then the same plate showed
fatigue after exposure to light. Another proof is that the
same plate that showed fatigue became active after being kept
in a dark place for a few hours. This last would also prove
that fatigue is not due to roughening. as nothing was done to
the plate to change its physical condition back to the starting
point. B. S. Allen says. - ”light is not the primary cause of
fatigue, though it may play a secondary part in accelerating
or retarting fatigue“.

Does fatigue depend on the sise of the containing
vessel? I will not discuss this question because it has no
bearing upon my work as I used no containing vessel. I will,
however, quote the conclusion of H. 3. Allen. - "The rate at
which the fatigue proceeds diminishes with the sise of the
containing vessel”.

Does photo-electric fatigue depend on the electrical.
condition of the plate? Ireueler, Hallwacks, Y. Schewidler and
Sodsewics all tried this experiment and found that the double
layer effect is not the primary cause but. may contribute in sons
degree, and that no connection could be nude between that contact
potential and fatigue. Having eliminated the first four possible

causes so far as ny work is concerned, it seems evident that the

-18..

fifth is the cause: at any rate most authorities agree that to
the best of their knowledge, it is the cause. It is a known fact
that a powerful source of ultra-violet light will produce ozone.
This was found by H. S. flower in the early ninties. H. S. Allen
nabs the following statement, «- ”A very powerful source of ultra--
violet light may cause an increased fatigue in consequence of the
production of ozone, which is one factor in bringing about a
diminution of the photo-electric current”. Winchester and Millikan
mks the following statement, with regards to the abscence of
fa-tigm in a high vaccum, - ”It further shows that the phenomenon
of photo-electric fatigue as ordinarily observed is one which has
its seat in the gaseous layer surrounding the metal rather than
in the metal itself”. With these three authorities arriving at
the same conclusion independently, we can, with confidence, assume
that photo—electric fatigue is due to the formation of a gaseous
layer on the metal. In my observations I would ear that photo-
electric fatigue has been realised. In one case where I was test-
ing redwood-carbon of grain number NO, (that is, it had passed
through a number no screen upon a number 60 screen), the tine to
pass over 10 scale divisions was 56.80 seconds, and after five
minutes exposure, not steady exposure but intermittent exposure,
the time increased to one minute nine seconds average. innother
case was with northern-pine carbon. The average time of discharge
of 10 scale divisions was In seconds and after 2 minutes exposure

to light the time increased, for the same discharge, to 27.50

«f,

seconds. Another case was a “Golden Brown Dye“ mmifactured by
the Monroe Drug Company. The average of b readings for a 10
scale division di schafge was 32.30 seconds, and the average of
the last six readings MJS seconds. No attenpt was nude to
determine whether or not they would revive, but from observation

when redwood-carbon was tried again I noted sane evidence of

recovery from fatigue.

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The Photo-electric Effect
of
Finely DiVided OarbOn
and
Certain Other Substances

The set-up to measure the photo—electric effect mist
consist of a source of ultra-violet light, and electrometer or
an electrosc0pe and a holder for the plate. If an electroscope
is used, a lens system including a source of light (other than
the ultra-violet), and a screen upon mich to project the imge
of the gold leaf of the electroscope. mist be used. The set-up
used is illustrated by a photograph opposit this page.

The sise of the plate for the various substances to be
tested is not inportant as long as it is small enough to have the
light fall upon it at near normal incidence. The size used was
5 x 6 cm. It is inportant, however, that the plate be not photo-
electric. several plates of different substances were tried and .
discarded, but common black stovepipe iron was found to be non-
photo—electric. This was true, however, when the oxide due to
its manufacturing process was still on the plate. When this oxide
was removed and the plate polished, it becane photo-electric.

It is inportant that the plate be very clean: all greases

and other foreign substances mist be removed. The first cleaning

-21..

process was to rinse the plate in hot water, then wash in hot
KOH, rinse thoroughly in.hot water, and then carefully dry.
This process was not successful because the plate became photo-
electric. This was due to one of two reasons, either a film.of
108 was still on the plate, or the oxide had been reduced and
the ultra-violet light passed thru it to the metal. To test
the first supposition the plate was rinsed in alcohol after it
had been rinsed in water the second time. This did not solve
the problem, but by careful examination it was found that the
oxide had been reduced. Then alcohol alone was used to clean
the plate and the same condition was found. It was thought
that perhaps some foreign substance in.the alchhol was the cause.
so redistilled alcohol was used with no change in results. Finally
hot water and Jap Rose soap was used and it was found that this
cleaned the plate and did not change it's photo-electric preperties.
The fact that alcohol could not be used, presented
another problenn It had been.p1anned to use alcohol as a solvent
for some of the dyes that were to be tested. It is necessary
when a.plate is prepared to have an evenly distributed film of
the substance on the plate, but due to the fact that alcohol
affected the plate it could not be used, After trying several
solvents. (acetone, alcohol, redistilled alcohol, hot alcohol,
hot acetone, water and hot water) to find one that would not

affect the plate but still give an evenly distributed film,

it was found that the plate could be prepared by dusting the
substance onto it. .After the plate had been cleaned, every
precaution was taken to keep it clean. The plate was tested
before the substances were dusted on and if found to be photo—
electric it was cleaned. If it again showed a trace, the plate
was discarded.

The ultra~violet light was obtained from a carbon arc,
using 3/8 inch solid carbon rods. The length of the arc is an
' important factor and this was controlled by measuring the volt-
age across the are. It was found that this method kept the
distance more constant than was possible by measuring. The
resistance was adjusted to give a.current of 27 auperes and the
arc adjusted so that the potential across it was “0 volts. The
source of current was not constant, therefore averages were
taken to reduce any errors.

The readings were taken for 10 scale divisions. They
are as follows: scale reading when ultra-violet light was turned
on, time for gold-leaf to pass over 10 scale divisions, scale
division when ultra-violet was turned off, voltage across arc
during this time. There was no particular reason for choosing
Just these values, but they were accurately kept.

There is no set standard used, so zinc was selected.
The reason for this choice is that zinc has a very marked.photo-
electric effect and the plate could be polished very readily.

There are two reasons for using a standard. First, the standard

-23-

plate can be used as a comparative value for all other substances.
Second, the polished plate can be preserved and used as a check
from time to time to insure accurate readings. A question might
arise here as to why we polish the plate. ‘Tirst, to remove all
oxides so that we have clean metal. Second, the contour of a
rough plate contains hills and valleys. so to speak, and we
polish the plate to make the surface as nearly plane as possible.

Before turning the ultrapviolet light upon the plate,
the system, (plate, connecting wire (H) and electroscOpe (E).
figure 3) was charged with a negative charge. Then the ultra-
violet light was turned on. If the substance under test was
photo-electric, the electroscope immediately began to discharge
with a definite rate. This operation was repeated several times
and readings taken for 10 scale divisions and the average taken.
The average for the standard.plate was found to be 5.007seconds
for a 10 division discharge. This average was obtained for one
hundred readings with a potential across the arc of “1.28 volts.
This then will be considered as the standard value and all other
values will be compared with it. '

An error that may creep in without being noticed is the
thermal effect, by this we mean the effect of temperature upon
various substances. If the system described above is charged
positively; substances sometimes show apparent photo-electric
effect. To avoid this error a test was made on each substance.

The test was a simple one. The system.was charged.positively

and the ultra-violet turned on and the time noted. In every case

no photo-electric effect was noticed for a 60 second exposure.

The Photo-electric Effect
Finely Divided Carbons

There is no chemical difference in the different carbons
found.in table I, but we list them.according to their source, keep-
ing in mind that they differ in origin and degrees of hardness.
Thus a carbon listed as ”redwood carbon" is carbon obtained by
partially burning redwood and grinding the residue in an agate
mortar.

The question may arise, “Is the carbon finely divided?"
Fig. h is s.photoemicr0graph of carbon that has been ground.in an
agate mortar. Fig. 5 is a photo-micrograph of carbon that has
' passed through a 100 mesh screen. The photo-micrograph Fig. 6
is carbon that was deposited.upon the plate by holding it at a
distance of eight inches over the flame of burning parafine.

The carbons were dusted.upon the plates and all excessive
particles were removed by Jarring the plate. In some cases it was
deemed advisable to brush or rub the particles on the plate. Wher-
ever this was done, rice paper or absorbent cotton was used.

It is evident that the harder the carbons the more active
the photo—electric effect, as is shown in table I. Cypress, paplar,

redwood, southern.pine and northern pine are of the soft grade of

carbon. They have a flaky and needle-like structure. Table I.
will show that they are not as active as parafine carbon, arc
carbon, chestnut, red-oak, black-walnut and gum-wood which are
hard and have a.distinctive granular structure.

It was further noted that the softer grades of carbons
would stick.to plate much better, thus more particles would be
deposited upon the plate per square centimeter. Naturally a
greater photo-electric effect was expected, however, the reverse
proved to be true. Therefore, the hard carbons are more active
than the soft carbons.

The size or the fineness of the carbons seem to modify
the photo-electric effect. Redwood carbon, A - table I, is carbon
ground in a porcelain mortar. It was coarse and.flaky. Note, it
is not very active, in.fact it is so slow that one ought almost
discard it. Note further from table I. that the finer the carbon
the more active the photo-electric effect. Redwood carbon, 100
fine, is twice as active as redwood carbon “0 fine. Parafine carbon
after it had been brushed with rice paper, was more active. There-
fore, from the data obtained, we have sufficient proof that the
fineness of the carbon does have an effect upon the photo-electric
effect.

The age of the carbon did.not seem.to here any effect
upon the photouelectric effect. Redwood carbon, 3 - table I, is
carbon one week.old, its activity is 0.19%. Redwood carbon 100

is fresh carbon, its activity is 0.168. The difference in activity
is too small to prove any change in the photo—electric effect due to

the age of the carbon.

Table I

Name . Note Scale Time of Voltage Comparison of

Division Discharge Across Arc Standard or
5.001

1

Parafine carbon 2 10 9.2% 111.100 0.5h2
- I 1 10 8.300 1+1.700 0.603
Parafins 1+ 10 —-——- 110.000 ----
Canphor gm 1 10 26.560 112.300 0.189
Poplar carbon 5 10 - 26.5hl ”1.200 0.189
Southern pine 5 10 23.770 111.1400 0.210
Bone black - 10 113.200 141.100 0.119
Northern pins 5 10 23.1110 15.800 0.216
Redwood A 10 70.870 1+1.h50 0.071
Redwood 100 7 10 29,731+ h0.000 0.168
Redwood 80 7 10 hit. 720 l+0.000 0.112
Redwood 60 7 10 66. 71:2 1:10.000 0.075
Redwood N0 7 10 6h.9N0 ”0.000 0.077
Redwood 3 10 30.900 140.000 0.1911
Cypress 5 10 13.710 “2.200 0.353
.Arc carbon 6 10 19.290 h1.000 0.261
Chestnut 5 10 17.726 no.636 0.283
Rod-oak 5 10 16.557 Mai? 0.302
(him-wood 5 10 19.210 ‘42. 1 ' 0.260
Black—walnut 5 10 19.000 110. 770 0.26M

Notes:

1. Brushed on plate with rice paper or absorbent cotton.

2. field over the flame at a distance of 8 inches.

3. Carbon.one week old.

11. Plate dipped in hot parafine and cooled.

5. Carbon ground in agate mortar and passed through 100 mesh screen.

Carbon collected at base of the arc and brushed on the plate with

rice paper.

7. “0 carbon means passed through number no screen upon a number 60 screen.
60 carbon means passed through number 60 screen upon a.nunber 80 screen.
80 carbon means passed through number 80 screen upon a number 100 screen.

100 carbon means passed through number 100 screen'upon.pan below.

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A Photo-electric Study
of
Certain Dyes

Are dyes photo-electric and does their position in
the spectmm indicate that they are photo-electric? In answer
to this, reference will be made to certain dyes that were tried.
A scarlet dye (Putnam) was tried and found to be photo-electric,
but not very active. (Table II) Near the scarlet in the spectmm
is the turkey red. Turkey red is darlser than scarlet but is such
slower, having a discharge time of 18.585 seconds. (Table II) The
next in position is yellow (Putnam). This is very slow, having a
discharge tine (10 divisions) of “3.68 seconds; it being only 0.115
as active as the standard. Bright green (Putnam) compares favorably
with the scarlet. (Table II) The olive green (Putnam) is darker
green and is slower. (Table II) Navy blue (Putnam) is slower than
the standard, having a discharge time of 13.389 seconds. Dissolv'ing
the navy blue in alcohol, it was found that the photo-electric effect
(Table II) is very active. The inaccuracy of this test is the effect
of the alcohol upon the plate as explained on page 22. Indigo (Dow)
is very active (Table 11) being 0.782 as active as the standard.
Purple violet (Putnam) is somewhat slower than indigo, hit compares

favorably with the standard (Table II).

.27-

Jifu

Table I;

Scarlet (Putnam)
Turkey Red (Putnam)
Yellow (Putnam)
Bright Green (Putnam)
Olive Green (Putnam)
Navy Blue (Putnam)

Navy Blue (Putnam - dissolved
in alcohol)

Indigo (Dos)
Purple Violet (Putnam)
Scarlet (Putnam)

Indigo (Dow - dissolved
in alcohol)

Scale

Division Discharge Across

10
10
10
10
10
10

10
10
10
10

10

13.h15
18.585
n3.680
1h.91h
29.557
13.389

8. 787
6.1m
1n.91u
5.180

3.720

Arc

No.615
MO. 285
h0.ooo
“3.500
MI. 711+
1+1. 536

141.333
h3.soo
t3.500
h1.200

h0.000

Tina and Voltage Comparison

of Standard

5.001
1

0.373
0.268
0.115
0.336
0.171
0.37“

0.569
0.732
0.725
0.91u - r

1.3h6

The first part of the (pestion is answered - dyes are;
photo—electric. The second part seems doubtful, as the data ob-'
tained for dyes in the red and of the spectrum, do- not seem to be
constant, but from data of the dyes in the blue and of the spectmm,
it seems that as the dye approaches the shorter wave-lengths the

photo-electric effect increases.

The Photo-electric Effect
Miscellaneouzfaibstancss
Metals have been tested many times and little will be

said here except to call attention to a few conditions that were
tested. Tinfoil was tested and found to be photo—electric.
(Table III) Then the same plate was moistened with distilled
water and the photo-electric became more active. a cappsr plate
was tested, the oxide was removed and the plate polished, and as
expected it was found to be photo-electric. This test was used
as a conparison. A thin film of copper was electrOplated upon
the polished surface and it was found that the photo-electric
activity had been greatly reduced. The plate was then polished
and it was found to be more active. However, test (c) did not
prove to be as active as test (a) which was due to the fact that

the cepper plate was softer and could not be polished as highly

.23-

as the original plate. (Table III - conditions (a), (b) and (c)
.A brass plate was then tried and found to be very active. (Table
III) The best results were obtained fromta tinjplate. The plate
was cleaned and tested and it was found to be very close to the
standard plate, having a discharge time of 5.106 seconds.

A.plate was made of chlorophyll. .1 solution of chlor-
Ophyll was obtained by dissolving a catulpa leaf in hot alcohol.
This solution was then pored.upon.a plate in such.a.nanner that
a uniform.film'was produced; then the alcohol was allowed to
evaporate, leaving a film of chlorOphyll. The plate was not
very good as it was almost impossible to get a very thick film
of chlorophyll on it. Table IV shows that it is not active.
Then catupla, maple and elm leaves were tried and it was found
that they also were not active. Note Table IV, that the time
for 1 scale division discharge was “2 seconds, for'the elm.1eaf.
From the data in this table it is believed that leaves are not
photonelectric.

A.plate was prepared by heating iodine and allowing
the vapor to condense upon the cold.plate. From.flhis test it
was found that iodine was photo-electric but not very active.
(Table IV)

a plate was prepared by molding a plate of sulphur

5 x 6 cm. and tested. It was found to be nonephoto-electric.

(Table IV)

-29-

The standard plate was again tried but this time a
quartz window was placed in the light stream and from Table V
will be seen that the photo—electric effect was changed but
very little. A thin photographic film was used as a filter
and from Table 7(a), it will be seen that the photo-electric
effect was greatly reduced. Then another film, of double
the thickness of the first, was tried and the effect was reduced,
still more. Then a film of three times the thickness of the
first was tired and the effect was reduced a great deal more.
These films were prepared by exposing photographic films

5, 10 and 15 seconds.
The effect of ray filters upon the photo-electric

effect was next studied. A No. 6 Eastman ray filter (light
yellow), No. 9 Eastman ray filter (dark blue), No. l nastmn
ray filter (dark red) were tested. No effect was observed
with No. 6 or No. 9, but with the No. 1 a reading of 105.5
seconds for 1 scale division was obtained, which was opposite
to what may have been expected. nore light passed through
the No. 6 filter and light of shorter wave-lengths passed
through the No. 9 filter. This test was repeated within one-
half an hour and the readings checked within 0.6 seconds.

I Therefore, carbons are photo-electric; the photo-
electric effect depending upon the fineness and hardness of
the carbon particles. Dyes are photo-electric and the effect

increases as we go from the red toward the violet dyes. Certain

natals are more photo—electric than other substances, the
photo-electric activity of any one of them varying with the
degree of polish. Leaves are photo—electric. Sulpher is
not photo-electric. Iodine is slightly photo-electric, but
not very active.

In general, most substances are at least slightly
photo-electric. The substances that have been proven to be
non-photo-electric might possibly be slightly photo-electric

if tested under different conditions.

Hans

Tinfoil
Tinfoil a 320

Copper -'polished (a)
(not elect roplate)

Copper - rough (b)
(electroplate)

Copper - polished (c)
(electroplate)

Brass

Tin Plate

Table 1;;

Scales

10
10
10

10

10

10
10

31.1%
30.940
7. 300

22.300
9.220

6. 960
5.106

Time and Voltage
Division Discharge Across

Arc

110.000
No.000
lu.060

10.000
111.200

No.30
l10.130

Conparison
of Standard

5,001

Table IV

Nam Scales Time and Voltage Conparison
Division Discharge Across of Standard
m 2-391
Iodine 1o 36. 787 10.000 0.137
801er 10 --——- 140.000 -....
Chlorophyll 10 78.181 1.2.272 0.0611
llm leaf 1 29.500 112.000 0.017
Catulpa leaf 1 33.200 140.000 0.015
Maple leaf 1 28.000 1+1.500 0.018

Table V

lass Scales Time and Voltage Comparison
Division Discharge Across of standard
Arc 5,001
1
Standard 10 5.007 1+1.280 1.000
standard - with quarts 10 6.000 1.0.000 0.835
Standard (a) 1 617.100 110.000 0.008
Standard (b) 1 70.600 140.000 0.067
Standard (c) 1 81.1100 h0.000 0.006
Note:

(a) - A photographic film exposed 5 seconds.
(b) - A photographic film exposed 10 seconds.
(c) - A photographic film exposed 15 seconds.

on

Bibl iogrghy

Physical Boview - Vol. 22
Photo-electric Pmperties
1. Aluminum - Dean Long, p. 525 (A)
2. Al‘genite; s M r m. Length Sensitivity - p. 1+61
Explanation of Photo Conduction - p. 529
Physical Review - Vol. 27
Effect of Absorbed Hydrogen and other gases on the Photo-
electric Activity of Metals - p. 267-281, V. L. Christen

Physical Review - V01. 28
Photo-Active Cells with Florescent electrolyte - p. 2'], p. Hodge

Physical Raview - Vol. 29
1. The dependence of the light intensity on Photo-
electric Current - p. 71, Richtmyer
2. Laboratory Application - p. 562
3. The dependence of Photo-electric on wave-length
. p. 561. Richtnyer
4. Effect on Alkali Metals - p. ltd-F, Richtnyer
5. Photo-electric Pr0perties - p. 17“. J. Kunts

Physical Review - Vol. 30
Photo-electric Properties of Alkali Metals - p. 39“ 8: 385

Physical Review - Vol. 32
Photo-electric Potential of thin cathod films - Paul Dike, p. 631
($89 Vol. 31" "' Po M’59)

Physical Review . Vol. 314
The Effect of a Magnetic Field upon Photo-electric Emission,
' A. W. Hull. p. 239

Physical Review - Vol. 1-52

1. The Velocity of Electrons in the Photo-electric
effect as a function of the wave-length of the
light - p. 16, D. w. Cornelius

2. Rectifying Properties of a Photo-electric Cell -
Po 222. Se He Anderfion

3. Conditions of sensibility of Photo-electric Cells
with alkali metals and hydrogen - p. 271+, J. G.

Kemp.

Physical Review - Vol. 11.52
Photo-electric Properties
1. A study of contact potentials and metals in a
mono; and the mitual relation between these
Phenomenon - p. 228
2. The Asymetric Emission of Photo—electrons from
thin films of Platinum - p. 195, Otto Stuhlmn

Physical Review - Vol. 6-52
1. New test of the Photo-electric equations
(Einsteins) - p. 55
2. A null method of photo-electric cells - p. 66,
Richtnyer

Physical Review - Vol. 7-62
1. The construction of a causative Photo-electric
cell - p. 62. Jakob mint
2. Sensative Photo-electric cells and a Photo-electric
relay - p. 282, Jakob mus, Job Stebbins
3. Einstein's photo-electric equation and contrast
electromotive force - p. 18, R. A. Millikan

Physical Review - V01. 8-52

1. The photo-electric current as a function of the
angle of emission and the thickness of the emitt-
ing film - p. 66, Willard Gardner

2. The influence of occluded gases on the photo-
electric effect - p. 238, R. J. Piersol

3. The normal photo-electric effed’. of lithium, sodium
and potassium as a function of wave-length and in—
cident energy - p. 310, W. H. Soodar

Physical Review - Vol. 9-52
1. The passage of photo-electrons through metals -'p. 558
Xe To Coup tan
2. Tolmans transformation equation and the photo—electric
effect - p. 290, S. Karru

Physical Review - Vol. 10-52
1. The theory of emission of the photo-electrons from
film coated and non-houegeneous surfaces - p. 78,
A. E. Hennings
2. The use of a Thomson galvanometer with a photo-
electric cell -‘p. 97
3. The emission of electrons in the selective and normal
photo-electric effects - p, N90, A. L. Hughes

Physical Review - Vol. 11-82
1. Photo-electric effects on mercury droplets - p. 276,
J. B. Deriexex
2. Preliminary Results in a determination of the maximum
emission velocity of the Photo-electrons from metal
plates at X-ray frequencies -'p. 505, Kang-Fuh-Ru

Physical Review - Vol. 12-52
The variation of the photo-electric current, due to heating the
occlusion and emission of gases -'p. 251, L. A. flelv

Physical Review - Vol. 13-S2

1. Photo—electric experiments - p. 310, J. Runs

2. The ctral photo-electric sensitivity of molbdenite
p. 1 0, Coblents & Lang

3. Spectral photo-electic sensitivity of silver sulphide
p. 291, Coblents & Kohler

h. The effect of Crystal structure upon.phcto-electric
sensitivity -'p. 163, W. W. Coblents

Physical Review - V01. lN-SZ -— see index
Physical Review - Vol. 18-52 - Photo-electric cells
Physical Review — Vol. 20-32
1. The variation of the photo-electric effect
with the thickness of the metal - p. 65, Stuhlman
2. Photo-electric emission.variation of the contact
difference of potential with temperature in.potassium
and the accompanying change - p. 102, R. Ives

Physical Review - Vol. 3-53

1. The theory of hate-electric action and chemical
action - p. , 0. W. Richardson

2. The Illumination current relationship in potassium
cells - p. 68, H. Ives

3. The positive potential in the Photo-electric effect
p. 639 We Be Kfida'Ch

h. The energy of photo-electrons from sodium and
potassium as a function of the frequency of the
incident light - p. 367. w. K. Kadesch

5. Planks Radiation constant - p. 1+76

The Electric Phenomena - p. 127, Physics of the Ether - Preston

The Production of Ozone by a Photo-electric current in 0.
Physical Review - Vol. 23, 1906 - p. 251

Fatigue and Recovery of the Photo-electric Current
Physical Review - Vol. 25 - p. 81, W. F. Holman

The Electron-Millikan
Electricity and Magnetism - Starling, pp. 573-586 and Bus-51‘s
Photo-electricity - Hughes

The Atom and the Bohr Theory of its Structure - Erasers-Holst
Chapters V 8: VII

Photo—Electricity - Allen

 

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