448 Professor 0. W. Richardson [May 7^ 



shown that the formula expresses the results within the limits of 

 experimental error, over the range of temperature from 1050° K. ta 

 2o00° 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 evaporation is to cool the liquid which gives 

 off the vapour, owing to the latent heat of vaporization. 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 "Wheat- 

 stone's bridge. Minute changes in its resistance can thus be 

 measured. The bridge is balanced with the electrode surrounding the 

 hot wire negatively charged ; so that the thermionic current does not 

 flow. If I 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 corresponds to a reduction of the resistance of the hot 

 wire, and thus to a lowering of its temperature. By experiments of 

 this kind. Professor 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 ele- 

 mentary 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, whose 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's 

 bridge. When the hot wire is charged positively there is no electron 

 current to the strip, and the bridge is balanced under these con- 

 ditions. The wire is then charged negatively, so as to make the 

 electrons flow on to the strip. There is then an increase in resist- 

 ance, 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 



