Sig. Quirino Majorana on the Contact Theory. 243 



due to a real electromotive force at the contact has not met 

 with general acceptance among modern physicists. Many of 

 those who admit the existence of this force consider it to be 

 something quite distinct from the electromotive force which 

 exists in a voltaic cell. From the latter free electricity can 

 be obtained without doing external work, merely by allowing 

 chemical action to take place ; a metallic couple, however, 

 cannot supply electricity unless the capacity of the system be 

 altered ; and to do this work must be done in overcoming 

 the electric attractions between the layers surrounding the 

 metals. 



It is a natural law that no body exists which, when put 

 into communication with one of the elements of a copper-zinc 

 couple, can take from it the smallest quantity of electricity ; 

 this is owing to the fact that the new electromotive force of 

 contact thereby introduced is such as to satisfy this condition. 

 Expressed in other words this is Volta's law of successive 

 contacts. 



It has been pointed out by many able experimenters that the 

 electrical phenomena observed on making contact between two 

 metals and varying their mutual capacity, cannot be attributed 

 with absolute certainty to the supposed electromotive force 

 of contact. Pellat *, for example, who has made most exact 

 determinations by the usual method of varying the capacity of 

 two dissimilar conductors, in order to obtain free charges, 

 measures the electromotive force of contact which, assuming 

 it to exist, would give rise to these charges. When, however, 

 he comes to state whether the contact is really the seat of 

 this electromotive force he is preoccupied with the existence 

 of the atmosphere in which the experiments were made, and 

 expresses himself thus : — ■ 



" Two different metals metallically connected f are sur- 

 rounded when in a state of equilibrium by electric layers of 



different potentials It is extremely probable that 



the observed difference of potential between the electric 

 laj'ers surrounding two metals in metallic contact represents 

 also the difference of potential which exists between the 

 metals." It is " extremely probable," that is to say, it is 

 not absolutely certain. The observcition of electric charges 

 on the surfaces of metals may therefore be explained other- 

 wise than by an electromotive force of contact. 



* H. Pellat, "Differences cle potentiel des couches electriques qui re- 

 couvrent deux metaux en contact," Ann. de Chimie et de Fhysique, 

 t. xxiv. pp. 1-136 (1881). 



t It would be sufficient to say that they have been discharged to 

 earth. 



T2 



