Contact Theory of the Galvanic Cell. 405 



negative, but less so than zinc. If, however, the two metals are 

 plunged together into water, the copper, zinc, and water forming 

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

 electricity is observed on any part of the system." " If a piece 

 of copper be now joined to the zinc, it (the copper) will become 

 negative, and the other copper plate positive, the difference of 

 potentials being that due to the direct contact between the zinc 

 and piece of copper only, the water having the effect of simply 

 conducting the charge from the zinc to the copper plate and 

 maintaining them at one potential"*. 



The foundation for these statements is found apparently in an 

 experiment due to Sir W. Thomson. He finds that if half-disks 

 of zinc and copper be arranged under a movable metallic needle 

 maintained at a high positive potential, if they are connected by 

 a wire or by contact, the needle moves in such a way as to show 

 that the copper is negative and the zinc positive ; while if they 

 are separated by a slight interval and connected by a drop of 

 water, no difference of potential is observed. Professor Jen kin 

 also lays great stress on the fact that, whereas copper in contact 

 with zinc becomes negative, in a single cell with wires attached 

 it is the wire attached to the zinc that shows a negative poten- 

 tial. This he holds to be conclusive that the junction of the 

 wire with the zinc plate is the real seat of the electrical separa- 

 tion ; although he admits that there may be a slight difference 

 due to the liquid, and that different liquids may augment or de- 

 crease this difference. 



In another place he says : — <e If the voltaic theory of the 

 cell were absolutely correct, the electromotive force of the cell 

 would depend wholly on the plates in the electrolyte, and not at 

 all on the solution employed to connect them "f. But it has been 

 found that the potential series of the metals is slightly changed 

 by the solution employed to join the plates : in order to account 

 for this fact it is necessary to treat the voltaic theory as incom- 

 plete. He adds, however, that the potential series of the metals 

 for water, dilute acids, and ammonic chloride do not differ so 

 much as to invalidate the theory, although the series for alkaline 

 sulphides is quite different and anomalous J. 



* Electricity and Magnetism, p. 44. f Ibid. p. 215. 



% It may be remarked in passing, that this identity of the potential 

 series for different acids may perhaps arise from a different cause, and not 

 be altogether such a proof of the contact theory as Professor Jenkin con- 

 cludes it is. Andrews has shown that, when one metal combines chemically 

 with different acids, the amount of heat liberated is the same, or nearly so. 

 Hence, if the metals be arranged in the order of their heat-producing power 

 when combined with the same acid, that order will remain the same for 

 most other acids. But the order is quite different when the metals are 

 combined with sulphur or oxygen. It is true that this order is not the 



