of the Electromotive Forces in a Voltaic Cell. 55 



electricity is being pushed into the copper wire at the end 

 where it is connected with the copper plate, and at the other 

 end is pushed out of the wire into the zinc plate. If at the zinc 

 end, electricity can only get away by rising in potential, then 

 it rises in potential, but the energy, which there becomes 

 potential-energy, comes from the battery, and it is not 

 necessary to get a supply of energy from any outside source. 



Of course this is only an analogy, but when we speak of 

 electricity flowing in a wire, we are using an analogy which 

 is derived from electrostatic and hydraulic phenomena to 

 explain a phenomenon of an electrokinetic kind. The idea 

 of a tension-energy in electricity as distinct from a potential- 

 energy may be objected to as being new and unnecessary, 

 but in spite of our very exact knowledge of mechanics we 

 have the same sort of difficulty in many mechanical ar- 

 rangements unless we assume the existence of a tension- 

 energy. For example, suppose you have a winding-engine 

 and a long endless inextensible rope passing over pulleys, 

 horizontal in places, vertical in places, and coming back to 

 the engine again, and at one place there is a weight being 

 lifted. Now work is being done on that weight in lifting it. 

 But energy does not come from the outside into the circuit 

 there ; it comes from the winding-engine ; it comes somehow 

 through the rope. Energy disappears at the winding-engine 

 and appears as work done in the weight being lifted. In 

 what form is it before it is converted into potential-energy ? 

 The energy is conveyed. In what form is it when it is in 

 the rope ? We really do not know, or, rather, we have not 

 given a name to it. But if we must give a name to it, it is 

 tension-energy. We know that there is a difference in the 

 tension of the rope above and below the weight which is 

 being lifted. Difficulties of this kind are really due to the 

 fact that the notion of conveyance of energy from one point 

 to another has not yet been co-ordinated with the ordinary 

 exact definition of energy. The energy which we usually speak 

 of as being possessed by a portion of matter, or as existing 

 at a particular place, is really possessed by a much larger 

 system. 



The analogy, however, which Dr. Lodge has used for 

 showing, as he considers, the absurdity of our position, is the 

 hydraulic one, but even with the analogy, when properly in- 

 terpreted, no such absurdity is found to exist. In fact the 

 explanation mentioned by Dr. Lodge in his paper as having 

 come through one of us from Sir William Thomson, but 

 which must really be regarded as having come from our- 

 selves, as we think that Sir William has probably his own 



