PHYSICS. 375 



the amount of energy which enters the circuit per unit of electricity 

 passing that point," and it is not possible to answer Lodge's question 

 in any other way than that in which he has himself answered it. The 

 real question at issue between them is quite another one. It is, is there 

 an actual contact difference of potential between zinc and copper? a 

 question which Ayrton and Perry answer in the affirmativ^e. (Phil. 

 Mag., January, 188G, V, xxi, 51.) In his reply Lodge confines his re- 

 marks to the thermo-electric views put forth in Ayrton and Perry's pai)er. 

 He thus restates their assumptions, every one of which seems to him 

 gratuitous and unsupported by experiment: (a) The characteristic 

 function of a simple thermo-electric circuit is an expression for the Volta- 

 effect between the metals of that circuit; (&) the Volta-effect of two 

 metals varies with the temperature ; (c) the total E. M. F. of a thermo- 

 electric circuit is equal to the difference between the Volta-effects of its 

 two metals at the temperatures of the hot and cold junction, respect- 

 ively ; {d) the specific heat of electricity falls greatly as it flows from 

 copper to zinc and rises a nearly equal amount as it flows from zinc to 

 acid ; (e) the rate of variation of Volta effect with temperature is a 

 measure of the Peltier-effect at a junction; (/) heat is generated or de- 

 stroyed at certain places in a circuit because electricity changes its ca- 

 pacity for heat there ; {g) reversible energy actions may go on in a cir- 

 cuit when a current passes without producing heat or any other form of 

 energy on the spot, and without either propelling or retarding the cur- 

 rent. To accord with his own view, the above statements should be for- 

 mulated as follows : [a) The characteristic function of a simple thermo- 

 electric circuit represents itself and no other physical phenomenon that 

 has yet been specially observed ; (6) the Volta-effecfc of two metals cer- 

 tainly varies with temperature if the heat tends to oxidize one metal 

 more than another, or in any other way to interpose a barrier between 

 metal and active medium, but the fact has no thermo-electric interest 

 whatever; (c) the total E. M. F. of a thermo-electric circuit is the sum 

 of the forces in the different parts of that circuit, viz, at the junction 

 and in the metals, and has nothing on earth to do with Volta's, or 

 Crookes's, or Hall's, or anybody's else '-effect," except Peltier's and 

 Thomson's ; {d,e) the Peltier-effect at a junction is a measure and con- 

 sequence of the E. M. F. located there ; (/, g) heat (or more generally 

 energy) is generated or destroyed at places where the current does work 

 or has work done upon it ; i. e., wherever it is opposed or assisted by an 

 E. M. F., and no^'here else. (Phil. Mag., March, 1886, V, xxi, 263.) 

 Ostwald, in a letter to Lodge, describes a method based on a statement 

 by von Helmholtz, by which differences of potential, whether between 

 two liquids or between a liquid and a metal, may be directly measured. 

 (Phil. Mag., July, 188G, V, xxii, 70.) 



Brown has communicated to the Royal Society an account of some 

 important experiments on contact action. He maintains that the differ- 

 ence of potential near tuo metals in contact is due to the chemical 



