198 Profs. W. E. Ayrton and John Perry. [Mar. 21, 



metal or electrolyte A, and the metal or electrolyte B (measured when 

 A and B are not in contact with other conducting substances), AB 

 being identical with — BA ; then the total electromotive force of any 

 closed heterogeneous circuit composed of the substances A, B, C, 

 N is :— 



AB + BC+ &c. +NA. 



The proof of this law is very important, as it is often denied without 

 experimental data. Profesor F. Jenkin says : — " The following series 

 of phenomena occur when metals and an electrolyte are placed in con- 

 tact : — 1. When a single metal is placed in contact with an electrolyte, 

 a definite difference of potentials is produced between the liquid and 

 the metal. If zinc be plunged in water, the zinc becomes negative, 

 the water positive. Copper plunged in water also becomes negative,* 

 but much less so than zinc. 2. If two metals be plunged in water (as 

 copper and zinc) the copper, the zinc, and the water forming a 

 galvanic cell, all remain at one potential, and no charge of electricity 

 is observed on any part of the system." 



It will also, we think, become evident that the natures of the 

 metals and of the electrolytes determine the electromotive force of a 

 voltaic cell, just as the natures of the metals and the distribution of 

 temperature determine the electromotive force of a thermo-electric 

 arrangement, and as the nature of the two metals zinc and copper de- 

 termines the electromotive force of Sir W. Thomson's mechanical cell 

 in which copper filings fall from a copper funnel through a zinc in- 

 ductor, in metallic connexion with the copper, into a copper vessel. 



II. 



At the surface of contact of two conducting substances there is an 

 electromotive force of definite amount which tends to make electricity 

 flow from one of the substances to the other. For instance, at the 

 junction of zinc and copper there is an electromotive force of 0"75 volt 

 which tends to cause electricity to flow from the copper to the zinc 

 across the junction. This flow ceases when the potential of the zinc 

 is 0*75 volt greater than that of the copper, the difference of potentials 

 of the metals balancing the electromotive force of the junction. It is 

 this difference of potential which is measured by the apparatus which 

 is described below. When a substance M is said to have a higher 

 potential than a substance N in contact with it, we mean that this 

 state has been established in virtue of an electromotive force which 

 exists at the junction of the substances, and that the electromotive 



* We found that copper is positive to distilled water, zinc being negative to the 

 water. 



