288 



SCIENCE 



[N. S. Vol. LIV. No. 1396 



tact. In the words of Volta, whicli I have 

 already quoted, " the metals can by themselves, 

 and of their own proper virtue, excite and dis- 

 lodge the electric fluid from its state of rest." 

 Admitting that the intrinsic potentials ex- 

 ist, a straightforward calculation shows that 

 they are intimately connected with the magni- 

 tudes of the thermionic emission at a given 

 temperature. The relation is, in fact, gov- 

 erned by the following equation: If A and B 

 denote the saturation thermionic currents per 

 unit area of the bodies A and B respectively, 

 and V is the contact potential difference be- 

 tween them at the absolute temperature T, 

 then V^JcT/e log A/B where k is the gas 

 constant calculated for a single molecule 

 (Boltzmann's constant), and e is the electronic 



I have recently, with the help of Mr. F. S. 

 Eobertson, obtained a good deal of new inform- 

 ation on this question from the experimental 

 side. We have made measurements of the con- 

 tact potential difference between heated fila- 

 ments and a surrounding metallic cylinder, 

 both under the high-vacuum and gas-free con- 

 ditions which are now attainable in such appa- 

 ratus, and also when small known pressures 

 of pure hydrogen are present. As is well 

 known, both contact potentials and thermionic 

 emission are very susceptible to minute traces 

 of gas, but we find that under the best condi- 

 tions as to freedom from gas there is a con- 

 tact potential of the order of one volt between 

 a pure tungsten filament and a thoriated fila- 

 ment. We have also been able to measure the 

 thermionic emissions from the filaments at 

 the same time, and we find that the contact 

 potential calculated from them with the help 

 of the foregoing equation is within 20 per 

 cent, of the measured value. Considering the 

 experimental difficulties, this is a very substan- 

 tial agreement. Whilst the evidence is not 

 yet as complete as I hope to make it, it goes 

 a long way towards disproving the chemical 

 view of the origin of contact potential differ- 

 ence. 



From what has been said you will realize 

 that the connection between contact potentials 

 and thermionic emissions is a very close one. 



I would, however, like to spend a moment in 

 developing it from another angle. To account 

 for the facts of thermionic emission it is nec- 

 essary to assume that the potential energy of 

 an electron in the space just outside the emit- 

 ter is greater than that inside by a definite 

 amount, which we may call w. The existence 

 of this w, which measures the work done when 

 an electron escapes from the emitter, is re- 

 quired by the electron-atomic structure of 

 matter and of electricity. Its value can be 

 deduced from the temperature variation of 

 thermionic emission, and, more directly, from 

 the latent heats absorbed or generated when 

 electrons flow out of or into matter. These 

 three methods give values of w which, allow- 

 ing for the somewhat considerable experimen- 

 tal difficulties, are in fair agreement for any 

 particular emitter. The data also show that 

 in general different substances have different 

 values of w. This being so, it is clear that 

 when uncharged bodies are placed in contact 

 the potential energies of the electrons in one 

 will in general be different from those of the 

 electrons in the other. If, as in the case of 

 the metals, the electrons are able to move 

 freely they will so move until an electric field 

 is set up which equilibrates this difference of 

 potential energy. There will thus be an in- 

 trinsic or contact difference of potential be- 

 tween metals which is equivalent to the diiler- 

 ence in the values of w and is equal to the 

 difference in w divided by the electronic 



PHOTOELECTRIC ACTION 



We have seen that there is a connection 

 on broad lines between thermionic emission 

 and both contact potentials on the one hand 

 and photoelectric emission on the other. The 

 three groups of phenomena are also related in 

 detail and to an extent which up to the present 

 has not been completely explored. In order 

 to understand the present position, let us re- 



" This statement is only approximately true. In 

 order to condense the argument certain small effects 

 connected with the Peltier effect at the junction 

 between the metals have been left out of consid- 

 eration. 



