434 Prof. Thomson on the Mechanical Theory of Electrolysis. 



nations to take place without any evolution of heat in their own 

 localities ; and it may be stated in general terms thus : — 



The intensity of an electro-chemical apparatus is, in absolute 

 measure, equal to the mecfianical equivalent of as much of the che- 

 mical action as goes on with a current of unit strength during a 

 unit of time. 



10. When o) is less than XI, 7 is (§ 8) negative ; and hence 

 equations (3), (5) and (6), show W and Al to be negative also. 

 In this case the direction of the current is contrary to the electro- 

 motive force of the disc ; the chemical action is the source of 

 the current instead of being an effect of it ; and the disc by its 

 rotation produces mechanical effect as an electro-magnetic engine, 

 instead of requiring work to be spent upon it to keep it moving 

 as a magneto-electric machine. If we assume 



7=-y, M=-M', W=-W', 



so that when 7, M, and W are negative their absolute values 

 may be represented by y, M' and W, we find by (9), (10), (5), 

 (6), (2), (3) the following expressions for these quantities : 



JR 



y=-^^(Xl-a,) (13) 



M'=j(9€y=:i-^^^:^(a-a,) . . . (14) 



W = M'-JH = ^r2Fa).y=g]Vf. . (15) 



The first of the three expressions (15) for W merely shows 

 that the mechanical effect produced by the disc in any period of 

 time is less than M', the full mechanical equivalent of the con- 

 sumption of materials in the electro-chemical apparatus, by the 

 mechanical equivalent of the heat generated in the whole cii'cuit 

 during that period. From the third we infer, that the fraction 

 of the entire duty of the consumption which is actually performed 



by the engine is equal to jr. If (o were precisely equal to XI, 



the electro-motive force of the battery would be precisely ba- 

 lanced, and there could be no current, and hence the performance 

 of the engine cannot be perfect ; but if (o be less than X2 by an 

 infinitely small amount, the battery will be allowed to act very 

 slowly j a veiy slight current, with a very small consumption of 

 materials, will be generated; and the mechanical effect produced 

 from it will be infinitely nearly equal to the whole duty, and in- 

 finitely greater than the portion of the effect wasted in the crea- 

 tion of heat throughout the circuit. 



11. A condition precisely analogous to that of reversibility, 



