470 
NATURE 
[JUNE 24, 1915 



exhibit a rather wide variation, which, however, is 
believed to be fortuitous; so that the mean value 
should be much more accurate. The mean of the 
eight values gives R=3-72 x 10°, whereas the number 
given by the gas equation is R=3-711 x10* in the 
same units. 
The fact that the value of the gas constant can be 
deduced in this way from purely electrical measure- 
ments must be regarded as a remarkable confirmation 
of the general position. The results of these experi- 
ments, and of others of a similar nature which | have 
not time to describe, show not only that the velocities 
of the electrons are distributed about the average 
value in accordance with Maxwell’s law, but also 
that the emitted electrons are kinetically identical 
with the molecules of a hypothetical gas of equal 
molecular weight at the temperature of the hot metal. 
The experiments referred to formed the first direct 
experimental demonstration of the truth of Maxwell’s 
law of distribution of velocities, and, although many 
of the consequences of this law have been made 
visible by the beautiful experiments of Perrin on the 
Brownian movement, I believe that they still furnish 
the most direct experimental verification of its truth. 
Quite recently a number of experimenters have 
called in question the general position which I have 
taken as to the nature of the process of electron 
emission from hot bodies, and have asserted that this 
effect is caused by chemical action between the hot 
solid and traces of contaminants, usually supposed 
to be gaseous, which have access to it. Whilst I 
feel that the value of the evidence in favour of the 
latter hypothesis has, generally speaking, been greatly 
over-estimated, it would take too long to discuss this 
question with the completeness which it demands. I 
shall therefore content myself with directing your 
attention to some experiments with tungsten filaments 
which prove that only an insignificant fraction, if any, 
of the emission from this substance can be attributed 
to chemical action. 
Tungsten is peculiarly suited to these experiments 
on account of its great refractoriness. It can be 
heated in a vacuum for considerable periods at tem- 
peratures so high that all known impurities are vola- 
tilised out of it. The preliminary treatment of the 
experimental lamps furnishes some novel features 
which may prove of interest. The ductile tungsten 
filaments are electrically welded to the supporting 
leading wires in an atmosphere of hydrogen. After 
mounting, the lamps are exhausted in a vacuum 
furnace (with an external air pressure of about 1 cm.) 
at 550-600° C. for about twenty-four hours, until 
the evolution of gas becomes very small. A Gaede 
pump is used for the internal exhaust at first, and, 
later on, liquid air and charcoal in addition. In the 
final stages the tungsten is glowed at about 3000° 
absolute, and, for the best results, the anode is heated 
by subjecting it to an intense electron bombardment 
from the hot wire. The conditions as to freedom 
from gaseous contamination which have been attained 
in this way are far superior to those which result 
from any other method of treatment. 
With lamps thus prepared I have carried out simul- 
taneous measurements of the rate of emission of 
electrons on one hand, and either of the variation of 
the pressure of the gas present or of the rate of loss 
of matter by the filament on the other. Particular 
experiments have led to the following numbers :— 
(1) For each molecule of gas given off the number 
of electrons emitted by the filament may be as high 
as 260,000,000. : 
(2) At each impact of a gas molecule ‘with the fila- 
ment 15,000 electrons would have to be emitted, and 
(3) Each atom of tungsten which disappears from 
NO. 2382, VOL. 95] 

the filament would have to cause the emission of 
984,000 electrons. ; 
The magnitude of these numbers entirely precludes 
the possibility that chemical action plays any signifi- 
cant part in this emission. Again, the mass of the 
electrons lost by a filament may exceed the mass of 
tungsten lost in the same interval, proving that the 
emitted electrons are not furnished at the expense of 
the tungsten. They must therefore flow in from out- 
side points of the circuit. Thus these experiments 
furnish a direct proof that the electric current in 
metals is carried by moving electrons. The mechan- 
ism of metallic conduction becomes more mysterious 
every day, but this, at any rate, is a fact which has 
to be reckoned with. 
Perhaps I can drive these matters home to you 
more effectually by means of a simple experiment 
which shows that these electron currents from tung- 
sten in high vacua are not minute affairs requiring 
elaborate apparatus for their detection, but, at high 
temperatures, are of such magnitude as to be worthy 
of the consideration of the practical electrician. 1 
have here a tungsten“lamp, containing a filament 
14 mm. long and about 3 mils. in diameter, in series 
with an ammeter, a resistance, a battery, and a 
second ammeter. They are arranged in the order 
named, so that there is an ammeter at each end of 
the lamp. In addition there is a side line from the 
cylindrical electrode of the lamp which can be switched 
through either a millammeter or an electric bell to 
the positive end of the battery. There is no auxiliary 
voltage in this side line. When I turn the current on 
you observe that the ammeters read differently, show- 
ing that more current is flowing into the filament at 
one end than out of it at the other. The difference 
is, in fact, equal to the electron current which flows 
into the wire sideways, and is registered by the mill- 
ammeter. Those of you who cannot see the instru- 
ments will, at any rate, hear the electric bell when 
I switch the electron current through it. With a 
lamp which was somewhat better designed for the 
purpose than the present one, I have recorded a cur- 
rent of 0-7 ampere at one end, 0-45 at the other, and 
0-25 in the branch circuit. So far as my experience 
goes, the only limit to the size of these electron cur- 
rents is that which is set by the magnitude of the 
current which fuses the filament, provided the requisite 
driving voltage is available. 
INTELLIGENCE. 
CamBripGE.—Mr. C. T. R. Wilson, F.R.S., Univer- 
sity lecturer in experimental physics, has been elected 
to a fellowship in Sidney Sussex College for a period 
of five years, 
Messrs. C. C. Bissett, of Emmanuel College; H. B. 
Cronshaw, of Gonville and Caius College; H. Ogden, 
of Emmanuel College; and E. P. Farrow, of Trinity 
College, research students. of the University, have 
qualified for the degree by the presentation of theses, 
which have been approved, in chemistry, mineralogy, 
physics, and botany respectively. 
The Harkness Scholarship. in geology and palwon- 
tology has been awarded to Mr. W. H. Wilcockson, 
of Gonville and Caius College, and the Wiltshire prize 
in geology and mineralogy to Mr. D. B. Briggs, of 
Jesus College. The Frank Smart prize in botany has 
been awarded to Mr. E. J. Maskell, Emmanuel 
College, and that in zoology to Mr. L. T. Hogben, 
of Trinity College. 
The following lecturers have been reappointed for 
five vears from October 1, 1915 :—Dr. Searle and Mr. 
C. T. R. Wilson, in experimental physics; Dr. Marr, 
