8 JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 



soft iron is subjected. I shall take the liberty of making some 

 remarks on this subject. In examining the phaenomena pre- 

 sented by the conjunctive wire^, Ave see that the beautiful theory 

 of M. Ohm completely accounts for them. Considerably en- 

 larged by the ingenious reseai'ches of M. Lenz, and conjointly 

 with the electro-chemical A-iews of Mr. Faraday, this theory has 

 become capable of connecting under one sole point of view a 

 multitude of facts. But, nevertheless, the elements serving as 

 basis to this theoiy are not placed beyond objection. The re- 

 sistance opposed by any conductor to the passage of the electric 

 current is there admitted as a permanent force, and enters as 

 such into the general expression of the force of the current. Let 

 E be the electro-motive force, R the resistance of a pair, audi?' 

 the resistance of the conjunctive wire, the force of the current, 



measm-ed in any way whatsoever, will be expressed by — -— = , 



and this force will increase indefinitely by multiplying at the 

 same time the surface n and the number 7n of the pairs. But it 

 would not be necessary to excite to a great degree the energy of 

 the battery, in order to destroy the conducting wire by the de- 

 velopment of heat, or rather by the heat which the wire itself 

 developes, in opposing the passage of the voltaic current. Cer- 

 tain powerful effects, which do not take place suddenly like other 

 physical pha;nomena, (for instance, the solidifying of water when 

 its temperature has sunk below zero), but which accompany all 

 electrical actions even from their most feeble indications, and 

 which are always directed towards the weakening of the con- 

 ducting power, must not be neglected when the nature of the 

 conducting wire has to be taken into account. ^I. Lenz, in 

 his valuable memoir on the conducting power of metals at va- 

 rious temperatures, has ckawn the attention of philosophers to 

 the complication of effects caused by the influence of the tem- 

 perature of the conducting wii-e ; the power of the current, the 

 temperatm-e, and the resistance being in an intimate and reci- 

 procal relation. In another memoir, this philosopher has an- 

 nounced some important facts relative to the conducting 

 poA\ er, which is changed by the least difference in the chemical 

 or physical conditions of the metals ; so that this power, mea- 

 sured AA ith precision, the type of which has been given by this 

 able physicist, may serve as the most delicate test of the purity 

 of the metals. I cannot, moreover, pass unnoticed the re- 



