440 Messrs. Trowbridge and Penrose on 



I will make one more remark, that when one seeks to form, 

 after the analogy of Hamilton's principle, that integral, taken 

 with respect to the time, whose variation gives the equations 

 of motion according to Maxwell in place of the electrodynamic 

 potential of Clausius (Maxwell's electrokinetic energy), «i bilinear 

 function of the components of the electric flow, on the one 

 hand, and of the components of the magnetic momenta, on the 

 other, arises in which the latter have to be dealt with, but not 

 as velocities. This last point I reserve to myself to treat soon 

 in another place. 



For the present I need only remind physicists that the 

 ground on which Prof. Clausius is inclined to reject the hitherto 

 accepted electrostatic system is a hypothetical assumption, 

 contested between different theories ; and I would beg them 

 not to transfer the name of the electrostatic system pro- 

 ceeding from Gauss, and hitherto employed by preference in 

 mathematical works, to another. In this system the potentials 

 (ra 2 /r ) and (e^/r) are quantities of work; their entire physical 

 importance rests upon the fact that they -are such. The theory 

 of the potential-functions forms one of the most complex and 

 interesting chapters of mathematical physics, corresponding to 

 well and perfectly known physical processes. If Gauss's units 

 be changed, then must we accustom ourselves to add factors 

 to all potential-functions, in order that they may remain quan- 

 tities of work and their differential quotients give the forces. 

 On the contrary, whether J 2 is a force and mJ a work, or 

 whether we must write for them A 2 J 2 and AmJ, appears to 

 me much less important, especially as we know well and 

 accurately just a portion of the department of electromagnetic 

 actions, viz. that consisting of closed currents, but in the pro- 

 vince of unclosed currents the most luxuriant flora of hypo- 

 theses still flourishes. 



XLIX. The Thomson Effect. By John Tkowbridge and 

 Charles Bingham Penrose*. 



SIR WILLIAM THOMSONf first discovered that when 

 an electrical current passes through a piece of metal, 

 the ends of which are of different temperatures, it carries 

 heat with it ; the direction depending upon the character of 

 the metal and the direction of the current. This pheno- 

 menon is known as the Thomson Effect. Le RouxJ subse- 

 quently verified Thomson's results, and gave an incomplete 

 table of the effect in different metals. No especial pains have 



* From Sillirnan's American Journal of Science for November 1882. 



t Phil. Trans. 1856, vol. iii. p. 661. 



% Ann. de Chim. et de Phys. 1867, [4] vol. x. p. 258. 



