244 Sig. Quirino Majorana on the Contact Theory. 



I do not intend to repeat here the history of the chemical 

 theory, but it appears to me interesting to recall that even 

 to-day there is one who holds similar views ; I refer to 

 Lodge *. According to this physicist the phenomenon is 

 very complicated and may be explained as follows: — A 

 substance immersed in any medium tends to exert a chemical 

 action on the medium (unless it is actually attacked). This 

 tendency keeps the substance at a potential different from 

 that of the medium in which it is immersed. This potential 

 is positive if the active element f of the medium is electro- 

 positive, and negative in the converse case. In addition to 

 this force of contact between the substance and the medium, 

 due to the potential chemical action, there is another which 

 is independent of chemical properties, which resides at the 

 contact of the two metals, and which, superposed on the first, 

 completes what is called the Volta efect. 



It is easy to perceive by what idea Lodge was influenced 

 in giving this explanation. Maxwell first stated that the 

 only direct method of measuring the electromotive force of 

 contact is by the Peltier effect. If A and B are two metals 

 an electromotive force A/B exists between them. An electric 

 current of intensity I passed first from A to B, and in a second 

 experiment from B to A, will develop in equal times different 

 quantities of heat Q ll and Q 2 , such that 



J(Qi-Q 2 ) = 2I(A/B), 



where J is the mechanical equivalent of heat. By means of 

 this relation we can obtain the values of A/B for the different 

 metals. They are, however, entirely different from those 

 measured electrostatically, both in magnitude and sign J. If, 

 therefore, the electromotive force at the contact of two metals 

 were similar to that which exists in a battery, we ought to 

 obtain the Peltier effect in the case of a dry contact, by a very 

 different method. Pellat concludes, from his important work, 

 that no relation exists between the difference of potential at 

 the contact of two metals and the electromotive force measured 

 by the Peltier effect. As I do not wish to depart too much 

 from the subject of this note, I will not dwell on the point 

 nor restate the reasonings by which Clausius and other writers 

 have endeavoured to reconcile these enormous experimental 

 divergencies. None of these reasonings lead us beyond the 



* 0. Lodge, " On the Seat of the Electromotive Forces in the Voltaic 

 Cell," Brit. Ass. Eep. 1884, pp. 404-529. See also Phil. Mag-, vol. xix. 

 (1885). _ 



t Ordinarily the active element is oxygen. 



1 Pellat, loc. eit. 



