OF WIRES FOR ELECTEICITY. 315 



that Becquerel arrived, from the above-mentioned experiments, at the 

 conclusion that they are in a direct ratio to the sections. 



Pouillet placed successively various wires between the poles of one 

 and the same pile, and determined the strength of the current by the 

 tangent of the angle of deviation. He asserts that he found the con- 

 duct! bility proportional to the section ; and with respect to the length, 

 he came to this interesting result, that the strength of the current is in 

 an inverse ratio to the length of the wires, if constant quantities are added 

 to them. If we denote the strength of two currents by F and F', the 

 corresponding lengths of the conducting wires by \ and \', and a con- 

 stant quality by /, Pouillet's formula will be 



F'_ l+\ 

 F / + \' ' 



but this is only the immediate consequence of our formula for the 

 strength of the current according to Ohm's theory, for according to it 

 we have for the conducting resistances X and X' (which are propor- 

 tional to the lengths), 



F = and F' = r ■ 



therefore 



F' _ / + X 

 F ~ / + X'' 



which exactly resembles Pouillet's formula, with the difference merely 

 that / has here a determinate signification ; it is, namely, the conducting 

 resistance of all the other parts of the circuit, except that of the wire 

 experimented upon. A very careful series of experiments referring to 

 the point in question is furnished us in a paper by Christie on the pro- 

 duction of currents by electro-magnetic induction by means of Knight's 

 great magnet. Totally unacquainted with the theory of Ohm, he was, 

 like Pouillet, induced, by an accurate discussion of his own observations, 

 to add to the conducting power of the wires employed in the experi- 

 ments a constant expressing the conducting resistance of the wire of the 

 multiplier. He must have obtained in this manner correct results, 



Da 



and was in fact led to the formula for conductibility -y , in which D 



signifies the diameter of the wire and L its length. Calculating by means 

 of this formula the angles of deviation, he found that the calculations 

 perfectly agreed with the observations made*. 



* We perceive in Christie's otherwise very valuable paper the want of a cor- 

 rect distinction between the electromotive power and the generated current ; 

 the latter being equal to the former divided by the resistance to conduction. 

 The electromotive power induced by the magnet in spirals, of any substance and 

 dimension whatever, isalways the same (see Memoires del'Academie de St. Peters- 

 hourg, — Sciences Mathem., vol, ii. p. 427,) whilst the current is in an inverse 



