468 
NATURE 
[June 24, 1915 

I have here an arrangement which will enable us to 
apply this test to the ions emitted by hot bodies. An 
exhausted tube carrying a horizontal hot wire is 
placed in a vertical electric field. The electric field is 
arranged so as to drag the negative ions emitted by 
the wire to a suitable electrode, whence they flow 
through a galvanometer, the deflection of which is 
registered by the spot on the screen. Around the tube 
an electromagnet is arranged, so that, when it is 
excited, there is a horizontal magnetic field which 
tends to curl up the paths of the ions. If I now 
switch on the electromagnet you observe that the 
current is at once reduced to a small value, showing 
that the magnetic field curls up the paths of the ions; 
so that they are now unable to reach the electrode. 
The carriers of this negative discharge are, in fact, 
electrons. 
I have here a second tube arranged to give a con- 
veniently large positive discharge. When this is tested 
by the electromagnet in a similar way, the magnetic 
field is found to have no influence on the thermionic 
current. The positive ions are, in fact, much more 
massive than the electrons; more elaborate experi- 
ments have shown that they are charged atoms. 
We see from these experiments that the negative 
emission is characterised by the electronic nature of 
the carriers and by its permanence in a vacuum. The 
presence of a gaseous atmosphere is not necessary in 
order to maintain these currents. Thus the electrons 
must come from the heated body itself. I believe 
that this emission is a process which is closely 
analogous to evaporation, The essence of evapora- 
tion, of a liquid, for example, lies in this: that, as 
the temperature is raised, the molecuies acquire suffi- 
cient energy to overcome the forces which attract them 
to the liquid, and so become free molecules of the 
vapour. We know that all material substances con- 
tain electrons, and it is not unreasonable to expect 
these to behave, when the temperature is high enough, 
in a way .analogous to the molecules of a liquid. 
Another analogy, in some ways more accurate, would 
liken the emission of electrons to the reversible evolu- 
tion of a gas by the decomposition of a solid such as 
calcium carbonate. The similarity of this process to 
evaporation is well known to chemists. 
This position is strengthened when we examine the 
way in which the electron emission depends on the 
temperature of the hot body. This may readily be 
done by surrounding the hot wire with a cylindrical 
electrode to catch the electrons, which then flow 
through a_galvanometer, the deflection of which 
measures their number. The hot wire is arranged 
to lie in one arm of a Wheatstone’s bridge, so that 
its temperature may be deduced from its resistance. 
Innumerable experiments with different substances 
have shown that this emission increases with great 
rapidity as the temperature rises, just as does the 
corresponding phenomenon in the case of evaporation. 
The correspondence is, in fact, exceedingly close. We 
may tale the rate of emission of molecules from the 
surface of an evaporating liquid to be proportional to 
the vapour pressure. The proportionality is not exact, 
but it is sufficiently so for our purpose. The crosses 
on the next slide represent values of the vapour pres- 
sure of water, on the vertical scale, plotted against 
the corresponding temperatures from 0° C. to 90° C., 
on the horizontal scale; whilst the circles represent 
the emission currents from platinum plotted similarly 
against temperature over the range t1o00° C. to 
1250° C. All the points lie on the same continuous 
curve within the limits of experimental error. To 
bring about this coincidence it is, of course, necessary 
to plot the temperatures on quite different scales in 
the two cases, but the agreement demonstrates in a 
NO. 2382, VOL. 95] 



simple way the similarity of the laws which govern 
the temperature variation in both cases. 
Numerous cases of electron emission have now been 
examined, and it has invariably been found, provided 
there is no reason to suspect changes in the chemical 
nature of the emitting surface, that the relation be- 
tween the current i and the absolute temperature T 
is expressed by a very simple equation. This is 
i=ATie-4/T, or logi—4log T=log A—b/T, where A 
and b are constant quantities for any particular sub- 
stance. The theory underlying this equation shows 
that the quantity b is very nearly equal to twice the 
energy change, expressed in calories, when one gram- 
molecular weight of the electrons is emitted. Pursu- 
ing the analogy with evaporation, this quantity may 
be called the molecular latent heat of evaporation of 
the electrons. It is not, however, with the theory 
underlying this equation that I particularly wish to 
concern you now; but I do wish to impress the fact 
that this formula is not an empirical affair covering 
a small range of temperature and current. The most 
recent measurements, made with tungsten, have 
shown that the formula expresses the results within 
the limits of experimental error, over the range of 
temperature from 1050° K. to 2300° K. At the lowest 
temperatures the currents were less than one-millionth 
of a microampere per square centimetre, and.had to 
be measured with a sensitive electrometer, whilst at 
the highest temperatures they were comparable with 
one ampere per square centimetre, and could be 
measured on a commercial ammeter. Thus the 
equation holds true, whilst one of the variables 
changes by the enormous factor of 10!. There are 
not many physical laws which will stand so severe 
a test as this. 
Let us now turn to some other consequences of the 
hypothesis that the emission of electrons is analogous 
to evaporation. One of the familiar effects of evapora- 
tion is to cool the liquid which gives off the vapour, 
owing to the latent heat of vaporisation. In an 
exactly analogous manner a wire which is giving off 
electrons will be cooled thereby. I think I can succeed 
in demonstrating this effect to you, although the 
lowering of temperature to be looked for is not very 
large, and delicate means have to be employed to 
detect it. This tube contains a hot tungsten wire 
which is made to act as its own thermometer by 
placing it in one arm of a sensitive Wheatstone’s 
bridge. Minute changes in its resistance can thus 
be measured. The bridge is balanced with the elec- 
trode surrounding the hot wire negatively charged, so 
that the thermionic current does not flow. ie 
reverse the potential and thus start the thermionic 
current, keeping the heating current constant, you 
observe a sudden deflection of the spot of the bridge 
galvanometer. The direction of this deflection corre- 
sponds to a reduction of the resistance of the hot wire 
and thus to a lowering of -its temperature. By 
experiments of this kind Prof. Cooke and I succeeded 
in measuring the latent heat of evaporation of the 
electrons directly. 
Just as a liquid is cooled by evaporation so it is 
heated to a corresponding extent when the vapour 
condenses. In fact an elementary experiment with 
which every student of physics is familiar consists 
in measuring the latent heat of evaporation by blowing 
steam into water. A precisely analogous experiment 
can be made with electrons. A large electron current 
from a hot wire is driven on to a fine strip of the 
metal of which the latent heat of condensation for 
electrons is to be tested. The cold strip is made to 
act as its own thermometer by placing it in one arm 
of a sensitive Wheatstone bridge. When the hot wire 
is charged positively there is no electron current to 

