88 



CASE OF THERMO-ELECTRICITY AND MACHINE ELECTRICITY. 



ther, and to expand, to a certain extent, OHM'S theory of the voltaic pile. It is a con- 

 dition, in tracing the action of wires of different lengths, to assume that the electromo- 

 tive power of the generating pair is under all circumstances constant, and hence it may 

 be conveniently represented by unity. But the electromotive power of any pair plainly 

 depends on two things: the quantity of electricity that the pair can evolve, and its abso- 

 lute tension. The theory of OHM, as may be gathered from the memoir of Professor 

 JACOB: on electromotive machines, and also from M. LENZ'S papers, confounds those 

 two important conditions. 



322. Now the results given in the foregoing table, proving that wires conduct in 

 the inverse ratio of their lengths, prove, also, that the addition of increasing lengths of 

 wire does not in any wise alter the electromotive power ; yet we have clearly shown 

 that this addition is inevitably attended with an increase of tension. Here, therefore, 

 is an apparent contradiction. 



323. But this contradiction is only apparent, and, when properly understood, leads to 

 a most remarkable result. 



324. It is true that we are compelled to assume that the electro-motive power of a 

 pair is independent of the length of the connecting wire ; but this constancy of electro- 

 motive power does not necessarily imply that the relations of quantity and tension, 

 which conjointly produce it, are not themselves variable. In the case before us, we 

 have direct proof that the tension increases, and also that the quantity decreases, as 

 the connecting wire becomes longer, and the converse ; yet the electromotive power 

 varying directly with them both, they must of necessity bear such a relation to each 

 other, that their product shall always be equal to unity. Hence we infer, 



325. That the law of MARRIOTTE in relation to the ponderable elastic fluids holds, 

 also, in the case of electricity developed by voltaic action, the elastic force or tension 

 of a given quantity being inversely as the space it occupies. 



326. The following table will at the same time establish LENZ'S law in the case of 

 thermo-electric currents, and prove that even in cases where the tension is so exceedingly 

 low. the elastic force of a given quantity of electricity follows the above-named law. 



TABLE L. 



The current here experimented with was generated by a pair of wires of copper and 

 tinned iron T V inch in diameter, and one foot long, the soldered extremity being im- 

 mersed in a bath of boiling water, and the free extremity carefully maintained at 59^ 

 Fahr. ; the third column in the table being calculated by the aid of the constant 1527. 



327. As respects electricity of high tension, a law extremely analogous to that here 

 indicated may be traced. The striking distance varies directly as the quantity accu- 

 mulates. If a given jar be successively charged with quantities of electricity, as the 

 numbers 1, 2, 3, 4, &c., the intervals of air through which the spark can pass vary di- 



