June 24, 1915] 
NATURE 
409 

the strip, and the bridge is balanced under these 
conditions. The wire is then charged negatively so 
as to make the electrons flow on to the strip. There 
is then an increase in resistance, due to the heat 
liberated by the condensation of the electrons, which 
is measured. In these experiments only part of the 
observed change of resistance arises from the effect 
under consideration. The remainder is caused by the 
kinetic energy given to the electrons by the auxiliary 
field used to drive them from the hot wire to the 
strip. This, however, is easily determined and 
allowed for. 
I have now indicated to you three independent 
methods of deducing the values of the latent heat of 
emission of the electrons. Let us see how the latest 
and most accurate values obtained by these methods 
agree with one another. The numbers found, and 
the names of the experimenters responsible for them, 
are shown in the following table :— 
Values of Latent Heat of Emission Reduced to 
Equivalent Temperatures. 
(1) From the temperature variation of the rate of 
emission :— 
Tungsten (Langmuir)... 
pe (KX. K. Smith) 
Platinum (various) 
10°5 x 10'—11'r x 107 calor ics Jur n 
10°94 x 103 ” » 
12 x 10'—16 x 104 » » 
(2) From cooling due to emission :— 
Tungsten (Cooke and 
Richardson) 11°24 x 103 ra if 
Tungsten (Lester) 11 04 x 108 aa 3 
Platinum (Wehnelt and 
Liebreich) - 13°9 x 10*—14°'5 x 108 of 50 
(3) From heating due to condensation :— 
Platinum (Richardson 
and Cooke) 13°5 x 104 55 os 
Unfortunately, the vacuum value for platinum given 
by the first method is still uncertain owing to com- 
plications caused by gaseous contaminants. Except 
for this the agreement between the different methods 
is very satisfactory. 
We come now to the very interesting question of 
the velocity and kinetic energy which these electrons 
possess when they are emitted. The fact that they 
are electrically charged enables us to find out a great 
deal more about their emission velocities than we 
can do in the corresponding case of the emission of 
ordinary molecules. By applying an external electric 
field we can influence the motion of the emitted 
electrons, and the precise nature of the effect exerted 
by the field depends on the velocity with which the 
electrons are shot off from the hot body. It is clear 
that we have no such method of controlling the 
motion of ordinary molecules. 
I shall now consider one of the arrangements 
which has been used in applying these principles to the 
analysis of the emission velocities. The hot emitting 
surface is a small strip of platinum, electrically 
heated, which lies at the centre of a much larger 
metal plate. The upper surfaces of the strip and 
the plate are flush with each other, and are main- 
tained at the same potential. Vertically above this 
lower plate and a short distance away from it is 
a parallel metal plate connected to the insulated 
quadrants of an electrometer. An arrangement is 
provided by which a suitable difference of potential 
can be maintained between the two plates so as to 
oppose the motion of the electrons from the strip 
towards the upper plate. It is clear that if the elec- 
trons have no velocity when they are emitted, any 
retarding field, however small, will be sufficient to 
stop them from reaching the upper plate and charg- 
NO. 2382, VOL. 95] 
ing up the electrometer. If, on the other hand, 
they are shot off with a definite component of velocity 
normal to the strip they will reach the upper plate, 
provided the corresponding kinetic energy exceeds 
the worl: they have to do to overcome the opposing 
difference of potential. Thus if the electrons are 
not at rest when they are emitted, they will give 
rise to currents capable of flowing against an applied 
electromotive force if this is not too large. I have 
here an arrangement, similar in principle to that just 
described, which will enable me to show to you the 
existence of these currents flowing against an applied 
electromotive force. The platinum strip is replaced 
by a very short tungsten filament, the upper plate by 
a surrounding cylinder, and the electrometer by a 
galvanometer. The apparatus is thus different in 
detail from that already referred to, but the principle 
is the same. You observe that the current is largest 
when the opposing difference of potential is zero, and 
falls off uniformly and rapidly as the potential differ- 
ence is increased. By increasing the temperature I 
can cause a considerable current to flow against an 
opposing difference of potential of one volt. 
The experiments just referred to. are a kind of 
electrical analogue of the high jump, in which the 
measuring tape is replaced by a voltmeter. Corre- 
sponding to each emission velocity there is a definite 
equivalent voltage. The fact that the current falls 
off continuously as the opposing voltage increases 
shows that the electrons are not emitted with a single 
velocity but with different velocities extending over 
wide limits. Careful experiments of this kind have 
enabled us to discover what proportion of them are 
shot off with velocities within any stated limits, to 
determine, in fact, what is the law of distribution of 
velocity among the emitted electrons. 
More than fifty years ago Maxwell concluded from 
rather abstruse theoretical considerations that the 
velocities of the molecules of a gas or vapour should 
not all be equal, but should be distributed in a certain 
way about the average value. This law, known as 
Maxwell’s law of distribution of velocity, is somewhat 
similar to that which governs the density of bullet 
marks on a target at different distances from the 
bull’s eye. The theoretical considerations which led 
Maxwell to establish this law for gases apply equally 
to the atmospheres of electrons outside hot bodies. 
Let us see whether the results of our experiments 
agree with Maxwell’s predictions or not. If the law 
of distribution of the normal velocity component for 
the emitted electrons is that given by Maxwell, it is 
necessary (and sufficient) that the currents i, and i, 
which flow against potentials v, and v,, respectively, 
should satisfy the equation 

= (v1 = ¥%) 
where R is the constant in the equation pu=RT of a 
perfect gas, and Q is the quantity of electricity which 
liberates half a cubic centimetre of hydrogen at 0° C. 
and 760 mm. in a water voltameter. The require- 
ments of this formula are found to be fully satisfied 
by the results of the experiments. Thus the 
logarithms of the ratios of the currents are found to 
be accurately proportional to the differences in the 
corresponding opposing potentials at a given tempera- 
ture. Again, since Q is a well-known physical con- 
stant, and the value of T was estimated during the 
experiments, we can use the experimental data to 
obtain a value of the gas constant R. Eight experi- 
ments, made under conditions as varied as possible, 
when treated in this way, gave values of R which 
varied between the extreme limits 308 x10* and 
| 4-46x 10" ergs per c.c. per degree C. These values 
log %4/ig= 

