338 TENTH ANNUAL REPORT OF 



The values of the electro-motive force of one and the same battery 

 are very nearly equal, although the nature of the liquid, and with it 

 the resistance to conduction, may change. In fact, the electro-motive 

 force of the Stohrer zinc and carl3on battery differs only 0.1 part from 

 the force of that constructed by Deleuil. This fact has already been 

 mentioned more at length above. 



It is now to be explained what we are to understand by these num- 

 bers. The electro-motive force is that iorce which sets the current in 

 motion. We can of course measure this force, as well as that of the 

 current, by its eifects. 



The electro-motive force of the voltaic pile is proportional to the 

 electrical tension of the pole in the open circuit ; we could, therefore, 

 apply this tension as a measure of the electro-motive force, if the 

 electrical tension were jiot so very small at the poles that it cannot 

 be determined with much accuracy in batteries of a few pair of 

 plates or elements. But Ohm's law teaches us that the force of the 

 current of the closed battery is also proportional to the electro-motive 

 force ; and since the power of the current can be measured with great 

 accuracy and reduced to a definitive unit_, it is better to use the force 

 of the current as a measure of the electro-motive force. We have 



w 



in which W denotes the entire resistance which the current has to 

 overcome ; when W = 1, we have 



E is here the force of the current which the battery would give if the 

 resistance to conduction were = 1. In establishing our units offeree 

 of current and resistance, let us consider the value of electro-motive force, 

 or the value of E, as the quantity of detonating gas which the current of 

 a battery would give if the ivhole resistance ivere equal to the resistance 

 of a copper wire 1 metre long and 1 millimetre thick ; thus if w^e have 

 iound the electromotive force E of Daniell's zinc and copper battery to 

 be 470, it means that the current of Daniell's battery would give 4*70 

 cubic centimetres of detonating gas per minute if the sum of all resist- 

 ance were equal to the above-mentioned unit of resistance. 



I consider it a great advantage of the chemical unit of force of cur- 

 rent recommended above (=: that current which yields one cubic cen- 

 timetre of detonating gas per minute) that in adopting it the values 

 of the electro-motive Iorce are not barely proportional numbers, but 

 that each has for itself a perfectly distinct and easily comprehended 

 signification. 



Although Jacobi was the first, to my knowledge, to attempt the 

 reduction of the data of the galvanometer to the chemical eifect, he 

 did not make any further use of this chemical unit of the force of the 

 current — that is, he did not apply it to the computation of the elec- 

 tro-motive force. 



§ 11. The electro-motive force is proportional to the tension of the open- 

 circuit. — It has been already mentioned that the electrical tension at 

 tue poles of an open battery may be considered as a measure of the elec- 



