Prof. Faraday on Electric Induction. 197 



equivalent of chemical action in the batteries, should the electro- 

 niotive forces be chemical) over and above that which they would 

 have had to spend in the same time, merely to keep up the cur- 

 rents, if the conductors had been at rest, because the electro- 

 dynamic induction produced by the motion will augment the 

 currents ; while on the other hand, if the motion be such as to 

 require the expenditure of work against electro-dynamic forces to 

 produce it, there will be twice as much work saved off the action 

 of the electromotive forces by currents being diminished during 

 the motion. Hence the aggregate mechanical value of the cur- 

 rents in the two conductors when brought to rest will be increased 

 in the one case by an amount equal to the work done by the 

 mutual electro-dynamic forces in motion, and will be diminished 

 by the corresponding amount in the other case. The same consi- 

 derations are applicable to relative motions of two portions of 

 the same linear conductor (supposed perfectly flexible). Hence 

 it is concluded that the mechanical value of a current of given 

 strength in a linear conductor of any form is determined by cal- 

 culating the amount of work against electro-dynamic forces 

 required to double it upon itself, while a current of constant 

 strength is sustained in it. The mathematical problem thus 

 presented leads to an expression for the required mechanical 

 value consisting of two factors, of which one is determined ac- 

 cording to the form and dimensions of the line of the conductor 

 in any case, irrespectively of its section, and the other is the 

 square of the strength of the current. If it be found necessary 

 to take inertia into account, it will be done by adding to this 

 expression a term consisting of two factors, of which one is 

 directly proportional to the length of the conductor, and inversely 

 proportional to the area of its section, and the other is the square 

 of the strength of the current. 



XXIX. On Electric Induction — Associated cases of Current and 

 Static Effects. By Professor Faraday, D.C.L., F.R.S. 



[The following important paper is a verbal copy of an abstract 

 of a lecture givcu by Professor Faraday at the Koyal Institution 

 on the evening of Friday, Jan. 20, of the present year. — Eds.] 



CERTAIN phajnomena that have ])resented themselves in the 

 course of the extraordinary expansion which the works of the 

 Electric Telegraph Company have undergone, appeared to me to 

 offer remarkable illustrations of some fundamental principles of 

 electricity, and strong confirmation of the truthfulness of the view 

 which 1 put forth sixteen years ago, respecting the mutually 



