Transient Electromagnetically Induced Current. 217 



the secondary circuit is less to the current induced by 

 the make than to the current induced by the break ; and 

 the time-integral of the former current is correspondingly 

 greater than the time-integral of the latter. 



Faraday in his first experiment found the current induced 

 by the make to be greater than that induced by the break, 

 but he explained it by the running down of his voltaic battery 

 during the time the current was passing through the primary, 

 in consequence of which the magnitude of the current stopped 

 on the break was smaller than that of the current instituted 

 on the make. This was undoubtedly a vera causa, and probably 

 one of considerable potency, considering that Faraday had 

 then no Daniel's battery and had no storage-cells to serve him 

 in his work. Another vera causa is the heating of the 

 circuit, which, even with a battery of constant E.M.F., may 

 render the current started very considerably greater than the 

 current stopped in ordinary experiments. Faraday, in his 

 original experiments^", had only magnetization of steel wires 

 to discover the induced currents by, and to test their magni- 

 tude ; and he had no galvanometer in the primary circuit. 

 If he had had a ballistic galvanometer in his secondary 

 circuit, and any suitable galvanometer for steady currents in 

 his primary circuit, he might possibly have found f that the 

 time-integral (as shown by the ballistic galvanometer) of the 

 primary current exceeded that of the secondary current by 

 a greater difference than could be accounted for by the 

 current being suddenly started and the current suddenly 

 stopped in the primary. So far as I know no one, from 

 1831 till now, has made any experimental examination of the 

 question suggested in Faraday's Exp. Res. Series I. 16 ; and 

 his idea that the two currents are equal has been generally 

 accepted | . I have therefore asked Mr. Tanakadate to make 

 some experiments on the subject in my laboratory. He 

 immediately obtained results § seeming to demonstrate a con- 



* Exp. Researches, Series I., 1831. 



t [No. On the contrary, he would have found that his first idea, of 

 perfect equality of the two currents, was perfectly true ! February 23.] 



X See Maxwell's 'Electricity and Magnetism' (1873), vol. ii. §537, 

 p. 171. 



§ But these results we find are quite untrustworthy because of the 

 susceptibility of the steel needle of the galvauometer to magnetic induc- 

 tion, which with the currents produced through its coil by the induced 

 currents due to the make aud break in the primary circuit, may largely 

 alter its effective magnetism. It is in fact well known that ballistic gal- 

 vanometers with steel needles give very erratic results if they are used in 

 attempting to find time-integrals of very intense transient currents of very 

 short duration. 



