In the fall of 1913, while studying the photo-electric properties of freshly cut surfaces of the alkali metals in extreme vacua, we observed that immediately after first cutting, the fresh surface of sodium showed very large photo-sensitiveness when tested with monochromatic light of wave length 5461 A., even when the vacuum was of the order 10(-6) mm. as measured by a McLeod gauge. But after several weeks of experimenting and many cuttings a condition was reached in which a freshly cut surface was completely insensitive when illuminated with this wave length. The lost sensitiveness reappeared, however, in the course of not more than two minutes after cutting, and grew rapidly to a very large value in fifteen or twenty minutes. When the gas pressure was of the order of 0.01 mm. the same phenomenon occurred but the rise to a maximum value was less rapid. From these results we began to surmise that photo-electric currents must be due to the influence of some active gas, which diffused from the walls to the metal and whose action upon the surface was retarded by the presence of an inert gas

Millikan, R. A.

Souder, W. H.

English

Caltech Authors - Main

PHYSICS: MILWKAN AND SOUDER
EXPERIMENTAL EVIDENCE FOR THE ESSENTIAL IDENTITY
OF THE SELECTIVE AND NORMAL PHOTO-
ELECTRIC EFFECTS
By R. A. Millikan and W. H. Souder
RYERSON PHYSICAL LABORATORY. UNIVERSITY OF CHICAGO
Received by the Academy. December 3. 1915
In the fall of 1913, while studying the photo-electric properties of
freshly cut surfaces of the alkali metals in extreme vacua, we observed
that immediately after first cutting, the fresh surface of sodium showed
very large photo-sensitiveness when tested with monochromatic light of
wave length 5461 A., even when the vacuum was of the order 10-6 mm.
as measured by a McLeod gauge. But after several weeks of experi-
menting and many cuttings a condition was reached in which a freshly
cut surface was completely insensitive when illuminated with this wave
length. The lost sensitiveness reappeared, however, in the course of
not more than two minutes after cutting, and grew rapidly to a very large
value in fifteen or twenty minutes. When the gas pressure was of the
order of 0.01 mm. the same phenomenon occurred but the rise to a maxi-
mum value was less rapid. From these results we began to surmise that
photo-electric currents must be due to the influence of some active gas,
which diffused from the walls to the metal and whose action upon the
surface was retarded by the presence of an inert gas.
While we were in the process of testing this hypothesis by repeating
the experiment under as diverse conditions as possible the papers of K.
Fredenhagen,l H. Kiister,2 G. Wiedmann and W. Hallwachs3 appeared
nearly simultaneously, all taking the point of view which had been
suggested to us by our own experiments, and the last one of them,
namely, that of Wiedmann and Hallwachs. asserting with consid-
erable positiveness that the presence of a gas is a necessary condition
for the appearance of photo-electric currents in potassium, and that the
"photo-electric effect in general takes place in the mixture formed by
the body and the absorbed gas."
By this time, however, we had obtained data which convinced us
that the point of view taken by these authors was untenable and in
April, 1914, at the meeting of the American Physical Society, we pre-
sented a paper entitled 'Effect of Residual Gases on Contact E. M. F.'s
and Photo-currents,'4 in which we described fully and exhibited to the
society the curves shown in figure 1. A glance at these curves shows
at once that the freshly cut surface is initially strongly photo-sensitive-
to lines 2535 and 2804 and that its sensitiveness for these lines diminishes
19
PHYSICS: MILUKAN AND SOUDER
continually with time; while for lines 3126, 3650, and 5461 the sur-
face is initially relatively insensitive and then grows in sensitiveness to a
maximum. Now according to Pohl and Pringsheim,6 the selective effect
in sodium shows a resonance band whose maximum is at about 340 ,upu.
It is obvious then that if we wish to define the selective effect as the
strong photo-sensitiveness in the neighborhood of this wave length,
then these curves mean that the selective effect so defined is due to the
presence of gas and grows as the gas has time to get at the surface.
Since we did not wish to define the selective effect in any such arbitrary
manner we interpreted our result in the following words:
A freshly cut sodium surface is not sufficiently electro-positive to respond
photo-electrically to light of wave-length 5461 A., but under the influence of an
T,me-Potoc±rre-nt Cr,,ues for Ditffertnt oYet L¢esgtths.
li*,t. . m .. |,:t . -t .i lA.:,r tt,r.tT- :
tzt I iii B 1 S 8ii» X Xttep ti
2-1t1 %:14.:!!KR 7 4jli. . S- 10NI2;
II 111 HI, IVJ,FIG. 1H
active gas, the sodium forms a new substance which is more electro-positive
than the freshly cut sodium and hence is photo-sensitive to longer waves.
The photocurve taken with wave-length 5461 A., represents then merely the
the freshly cut surface is itself so photo-electrically active that its own decay
curve completely masks the rise and fall curve due t6 the growth and decay
of the more electro-positive substance resulting from the action of gas upon
the sodium.
In view of these results the authors raise a question as to the correctness of
the conclusions of Wiedmann and Hallwachs that photo currents are obtain-
able only in the presence of gas. We suggest that if the Wiedmann and Hall-
wachs experiments are repeated with light of sufficiently short wave-lengths
instead of with visible light, it is not likely that the photo-sensitiveness willinstead light, 'I's likely photo-sensitiveness
20
PHYSICS: MILLIKAN AND SOUDER
be found to vanish in the way in which it did in the experiments reported by
these authors.
This article formed the subject of some correspondence between Pro-
fessor Hallwachs and one of us, and quite recently Wiedmann6 working
in Hallwachs' laboratory has followed out the suggestion above made and
obtained by cleansing his surfaces by successive distillation instead of
by cutting, as we had done previously, results altogether similar to ours,
though of less pronounced character and extending over a much smaller
range of wave-lengths. Wiedmann's actual observation is that potas-
sium, whose selective resonance band, according to Pohl and Pringsheim,
is at 436,Au, shows a relatively stronger photo-sensitiveness for line 365 pp
than for either 406 uA, or 436 ,Au,, when it has been freed from gas by
repeated distillation. But when distillations are less numerous and
freedom from gas less complete the photo-sensitiveness of this line is
some four times greater than that of the other lines. This means merely
in terms of our diagrams that lines like 2535 A. and 2804 A. which have
wave lengths shorter than that corresponding to the selective point are
relatively strong with respect to those whose wave-lengths are at or
above that of the selective point when gas has had little chance to act,
but that they become relatively weak after the gas had had a chance
to act.
These results from Professor Hallwachs' laboratory constitute then
very interesting confirmation of both the results and the conclusions
which we published one and one-half years ago, namely, that when
tested with sufficiently short wave-lengths, freeing a surface from gas does
not at all eliminate its photo-electric sensibility, though, as we pointed
out then, it does diminish the photo-sensitiveness on the long wave-
length side, i. e., at or above the region of the wave-lengths at which
Pohl and Pringsheim's selective effect reaches its maximum.
Within the past year we have made experiments which show that it is
very difficult if not impossible to set up any definite criterion for the se-
lective effect since the so-called normal and selective effects merge by
insensible gradations into one another. At present we prefer to regard
the selective effect as nothing but the normal effect taking place in the
neighborhood of an absorption band, i. e., in the neighborhood at which
oscillations of one particular frequency predominate. Our reasons for
this point of view are as follows:
(1) We have shown that the energy of emission is always governed by
the normal equation, -MV2=PDe =hv-P, whether the impressed fre-
quency coincides with the maximum of the selective effect or not.
21
PHYSICS: MILLIKAN AND SOUDER
(2) We have found that the angle of incidence or azimuth of polari-
zation is not at all essential to the appearance of a strong maximum at
the wave length at which Pohl and Pringsheim locate the frequency of
the maximum selective effect, for we find this strong maximum when we
work not only with unpolarized light but with normal incidence. In
other words, the only condition for the copious emission of corpuscles
is the coincidence of an impressed frequency with a natural frequency.
(3) We have found that with a clean surface the curve expressing the
relation between the frequency and the photo-current per unit of inci-
dent energy rises steadily with increasing frequency right through the
point assigned by Pohl and Pringsheim for the maximum of the selective
* - 4 mi. ftc.r renewing surfce(G08 prssure 0.04 mm.)
14- -bours after
./-' -- ~ - 43d13 dt.-a--ter
./ _ _ --134 days after
-o- \ ----
o/
FIG. 2
effect and that as a surface has opportunity to be acted upon by gas a
maximum slowly grows at the frequency indicated, and ultimately be-
comes very large. This means that if we regard the maximum as a cri-
terion of the selective effect, then the selective effect merges by insen-
sible gradations into the normal so that no sharp line of demarcation can
be set up between them. The results stated under (1) follow from the
photo-electric determination of Planck's 'h.'7 Those stated under (2)
and (3) follow from the data plotted in figure 2. This data was taken
as follows:
Light from a 220 volt quartz-mercury lamp was passed through a
Hilger monochromatic illuminator into a vacuum chamber in which the
22
PHYSICS: MILLIKAN AND SOUDER
alkali metals, lithium, potassium and sodium, were arranged on the
periphery of a wheel which could be rotated electro-magnetically by an
electromagnet outside the chamber. Fresh surfaces of these metals
were produced by means of a knife operated from outside by another
electro-magnet. A Hilger thermopile used in connection with a Cob-
lentz galvanometer sensitive to 1 X 10-11 amperes intercepted a portion
of the beam and permitted energy measurements to be made simul-
taneously with the measurements of the photo currents.
The apparatus was the same as that used in the photo-electric deter-
minations of Planck's 'h' with the aid of the alkali metals.7 As indicated
above, the incidence was always normal.
The photo-currents are all saturation currents with -12 volts applied
to the lithium.
The pressures at which the observation in the experiments corre-
sponding to curves in figure 2 were made ranged from0.04to 0.0001 mm.
While the rate at which the maximum grows depends somewhat up-
on the pressure, any individual curve is found at a given time to be not
essentially modified, so far as the phenomenon here under consideration is
concerned, by changes in pressure within the specified limits.
Changes in photo-sensitiveness bearing some similarity to these were
observed by Pohl and Pringsheim8 in the cases of barium, aluminium,
magnesium and calcium but the maxima observed by them were not
shown to be in the neighborhood of a wave length at which a selective
effect had been found. Indeed Pohl and Pringsheim did not interpret
their results as meaning that these four metals possess any selective
effect at all. It will be observed, however, that the maximum in curve
2 coincides, within the limits of accuracy of Pohl and Pringsheim's de-
termination, with the frequency 280 ,u at which they located the selec-
tive effect of lithium. It is not at all likely, therefore, that our normal
effect can differ in any essential particular from their selective effect in
lithium. It is true that we have not shown that our lithium surface shows
the characteristic of yielding an energy-frequency curve having a maxi-
mum when light is polarized perpendicular to the plane of incidence
combined with no maximum when it is polarized parallel to the plane of
incidence. But this was not the criterion which Pohl and Pringsheim9
used when they discovered the existence of the selective effect in lithium.
Their criterion was simply the existence of a pronounced maximum at
280 AjA when they used oblique incidence and polarized light. It is
scarcely conceivable that our maximum at 280 uAAu would disappear if
we made our tests under precisely the same conditions which they used.
Our experiments have, as yet, thrown no light on the cause of the ex-
23
GEOPHYSICS: L. A. BAUER
ceptionally profuse emission shown by potassium and sodium when the
electric vector of the light wave vibrates normally to the surface. But
so far as we can discover the selective effect in lithium has never been
shown to possess this characteristic.
Summary.-1. Photo-electric phenomena do not appear to be in gen-
eral conditioned by the presence of a gas, since they appear with the
use of sufficiently short wave lengths even when they do not appear
with the use of long wave lengths.
2. The long wave-length limit of photo-sensitiveness is modified by
the presence of a gas and this long wave-length limit is determined by
the presence of a substance which gives rise to a maximum photo-
sensitiveness at the so-called resonance wave-length.
3. Some of the characteristics of the selective effect have been obtained
under the conditions under which only the normal effect is commonly
observed.
4. The curves commonly supposed to be characteristic of the normal
effect may be made to merge by insensible gradations into those of the
selective effect by the action of gas on the surface of the metal.
5. All distinctions between the normal and selective effects in lithium
have disappeared.
I K. Fredenhagen, Physik. Zs., Leipzig, 15, 65 (1914).
H. Kilster, Ibid., 15, 68 (1914).
'G. Wiedmann and W. Halwachs, Berlin, Ber. D. physik. Ges., 16, 107 (1914).
4R. A. Millikan and W. H. Souder, Physic. Rev., Ser. 4, No. 1, 13, July, 1914.
6 R. Pohl and P. Pringsheim, Die li4htdektrischen Erscheinungen, pp. 14 and 27.
e G. Wiedmann, Ber. D. Physik. Ges., 17, 343 (1915).
7 R. A. Millikan, Physic. Rev., March, 1916.
8 R. Pohl and P. Pringsheim, Ber. D. Physik. Ges., 14, 546 (1912).
9 R. Pohl and P. Pringsheim, Ibid., 46 (1912).
CONCOMITANT CHANGES IN TERRESTRIAL MAGNETISM
AND SOLAR RADIATION
By L A. Bauer
DEPARTMENT OF TERRESTRIAL MAGNETISM. CARNEGIE INSTITUTION OF WASHINGTON
Read before the Academy, Novemnber 17. 1915. Received December 3. 1915
While good progress has been made by various investigators in estab-
lishing the relationship between fluctuations of the earth's magnetism
and those of the sun's activity during the sun-spot cycle, there are still
outstanding a number of important questions. The magnetic quantity
hitherto generally used, as for example, one of the magnetic elements
(chiefly the magnetic declination), or the range of the diurnal variation
24


Experimental evidence for the essential identity of the selective and normal photo-electric effects

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Experimental evidence for the essential identity of the selective and normal photo-electric effects

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