54 Professors Ayrton and Perry on the Seat 



he should make such an assumption. Putting on one side 

 the question as to what is exactly meant by electricity rising 

 in potential at a point, or by the potential inside a wire 

 through which a current is flowing, we are willing for the 

 sake of his argument to assume that in taking a unit of 

 electricity from a point in a section of the wire to the outside 

 and from that through the dielectric, and from that into the 

 wire to a point in another section, the work that must be 

 done is the same for any points in the sections as Dr. Lodge 

 assumes, and to take that work as the measure of the difference 

 of potential between these two sections; and still we ask 

 him why we must assume that, when electricity rises in 

 potential suddenly at a place in a circuit, energy must come 

 in from the outside to enable the electricity to rise in poten- 

 tial ? Suppose we grant that electricity gains in potential- 

 energy in flowing from copper to zinc, do we know so much 

 of an electric current as to assert that when the electricity 

 gains potential-energy it does not lose an equivalent quantity 

 of some other kind of energy ? Why may we not assume, 

 for example, that electricity loses tension-energy in passing 

 from copper to zinc, so that if it gains potential-energy it 

 loses tension -energy. From that point of view, part of the 

 function of the liquid in the cell would be taking electricity 

 from the zinc to the copper inside the cell, giving it tension- 

 energy, and this tension-energy is converted into potential- 

 energy when the electricity passes in the outer part of the 

 circuit from copper to zinc. 



But, indeed, there is no need to assume the existence of a 

 tension-energy. Why should we assume that work must be 

 done by outside agents at a place in raising electricity from 

 potential A to potential B ? By definition work will be done 

 if we take electricity through the dielectric from A to B, but 

 it does not follow that an outside source must do work in 

 carrying electricity from A to B through the wire itself. 

 We know so little of what is really meant by the electric 

 current, and the mechanical analogies which have been put 

 forward from time to time are so imperfect, not even excluding 

 the beautiful form that Prof. Poynting has given to Clerk 

 Maxwell's analogy, that we like to fall back upon the oldest 

 and simplest one, which is that, w r hen electricity is flowing in 

 a ware, at one end it is being continually thrust in, and at the 

 other end it is in consequence continually being thrust out, just 

 for the same reason that electricity tries to come to the out- 

 side of bodies — repulsion of electricity by electricity. Hence 

 in the case of a copper wire one end of which is joined to the 

 copper plate of a battery and the other to the zinc plate, 



