Contact Theory of the Galvanic Cell, 403 



or '7 of that of a DanielPs cell when the metals are perfectly 

 clean; but by oxidation of the copper it may be made equal to 

 or even greater than that of a DanielPs cell. He has also shown 

 that if zinc and copper cylinders be connected by a wire, the 

 electrometer detects a difference between the potentials of the 

 air in the interior, and, lastly, that if copper filings be allowed 

 to fall from a copper funnel in contact with a vertical zinc cy- 

 linder, they convey a negative charge to a receiver placed below. 

 Sir W. Thomson concludes that there is sufficient evidence to 

 show that zinc and copper attract one another chemically at any 

 distance if connected by a fine wire, and that, as Professor Tait 

 remarks, " when any two bodies of different kinds are brought 

 into contact, there is a certain amount of exhaustion of the po- 

 tential energy of chemical affinity between them, and that the 

 equivalent of this is, partly at least, developed in the new poten- 

 tial form of a separation of the so-called electric fluids, one of 

 the bodies receiving a positive, the other a negative change, 

 the quantity depending on the nature and form of the bodies"*. 



This is equivalent to saying that at the surface of contact 

 there is perpetually a force tending to separate the two electri- 

 cities in a direction perpendicular to that surface, while at all 

 points ever so little within it there is no such force. 



Professor Maxwell reiterates essentially the same facts. He 

 gives Thomson's proof that the electromotive contact-force at a 

 junction of two metals is represented by PJ, where P= the co- 

 efficient of the Peltier effect, or the heat absorbed at the junction 

 due to the passage of a unit of current for a unit of time ; and J 

 is Joule's equivalent. He remarks that the electromotive force, as 

 determined by this method experimentally, does not account for 

 the whole electromotive force of a simple couple. This latter is 

 in general far greater than that given by the Peltier effect for 

 the same pair of metals. " Hence the greater part of Volta's 

 force must be sought for, not at the junctions of the two metals, 

 but at one or both of the surfaces which separate the metals 

 from the air or other medium which forms the third element of 

 the circuit"j\ 



Professor Jenkin, referring to these experiments of Thomson, 

 adds that " In cases where no known chemical action occurs, as 

 where zinc and copper touch each other, and yet difference of 

 potential is produced, since this involves a redistribution of elec- 

 tricity, a small but definite consumption of energy must then 

 occur ; the source of this power cannot yet be said to be known" J. 



2. It seems to be universally admitted that when an insu- 



* Thermodynamics, p. 62, § 107. 



t Treatise on Electricity and Magnetism, vol. i. p. 302, 



X Electricity and Magnetism, p. 55. 



2D 2 



