to the Measurement of Electro-motive Forces. 557 



tive electrode,, and one with zinc, the other with platinum or 

 platinized silver for the positive electrode. The electro-motive 

 force of the latter cell would be sufficient to excite a current 

 through the circuit, but it might be found convenient to add 

 electro-motive force from some other source*. 



10. Prop. II. The resistance of a metallic conductor, in terms 

 of Weber' s absolute unit, is equal to the product of the quantity of 

 heat developed in it in a unit of time by a current of unit strength, 

 into the mechanical equivalent of a thermal unit. 



11. If H denote the quantity of heat developed in the con- 

 ductor in a unit of time, by a current of strength 7, the me- 

 chanical value of the whole effect produced in it will, according 

 to the principles established by Joule, be JH. But this effect 

 is produced by the electro-motive force, F, and therefore, by 

 Prop. I., we have 



JH = F7. 



Now, according to Ohm^s original definition of galvanic resist- 

 ance, if k denote the resistance of the given conductor, we have 



F 



If the electro-motive force and the strength of the current be 

 measured in absolute units of the kind explained above, the 

 number k, expressing the resistance in this formula, will ex- 

 press it in terms of the absolute unit introduced by Weber. 

 Using the value ky derived from this, for F, in the preceding 

 equation, we have 



This equation expresses the law of the excitation of heat in the 

 galvanic circuit discovered by Joule; and if we take 73=!, it ex- 

 presses the proposition to be proved. 



12. In Mr. Joule^s original paper on the, heat evolved by 

 metallic conductors of electricity f, experiment! are described, in 



* An examination of the thermal eflPects of a current through four equal 

 and similar vessels containing dilute sulphuric acid, and connected by means 

 of electrodes of zinc and platinum, varied according to the four permuta- 

 tions of double zinc, double platinum, zinc-platinum, platinum-zinc, in one 

 circuit, excited by an independent galvanic battery or other electromotor, 

 would throw great light on the theory of chemical electro-motive forces and 

 resistances. Vessels containing electrodes of other metals, such as tin, 

 variously combined, and direct and reverse cells of Darnell's battery, might 

 all be introduced into the same circuit. If the exteriors of all the cells 

 were equal and similar, the excesses of their permanent temperatures above 

 that of an equal and similar cell in the neighbourhood, containing no source 

 of heat within it, would be very nearly proportional to the rates at which 

 heat is developed in them. 



t Proceedings of the Royal Society, Dec, 17, 1840; Philosophical Ma- 

 gazine, vol. xix. (Oct. 1841), p. 260. 



Phil. Mag. S. 4. No. 14. Buppl. Vol. 2, 2 P . 



