140 Professor Faraday [June 8, 



series of short intermitting currents, rapidly recurring, pass through 

 it instead of one constant current. The outer coil, usually called 

 the secondary, has its terminations apart ; and these can be con- 

 nected either metallically by a wire, or arranged with any interval 

 or apparatus placed between them, in which effects of the induced 

 current are to be shown, and by which its characters are to be 

 examined. 



When the secondary circuit is metallically complete, each brief 

 current in the primary wire causes, according to well-known prin- 

 ciples {Exp. Res. 10, &c.), two successive currents in opposite 

 directions in the secondary wire ; and if a galvanometer be 

 included in the secondary circuit, it is seen that the communica- 

 tion of the primary wire with the battery is followed by a deflection 

 of the needle in one direction, which then gradually swings to and 

 fro, accompanied by curious spasmodic motions (which are under- 

 stood upon a moment's inspection), until it comes to zero : if the 

 primary current be then stopped, the galvanometer needle is de- 

 flected in the other direction, and after a few oscillations subsides 

 quietly to zero again. The sum of the alternate induction cur- 

 rents having been thus shown to be equal in effect to zero, it was 

 then explained how, if the secondary currents be interrupted in the 

 smallest degree, even by the intervention of a hair or a piece of 

 paper, all the currents of one kind, due to the beginnings of the 

 short inducing currents in the primary wire, were stopped off from 

 the secondary wire (being expended in the primary wire itself;, and 

 only those due to the cessations of the primary currents left to show 

 their power there ; so that the secondary wire could then give a 

 continuous series of intermitting (but not alternating) currents, all 

 of which, therefore, had a common direction. 



The remarkable character of the electricity of these currents was 

 then shown and explained. Its intensity is such that it can strike 

 across J or ^ an inch of air, whilst the intensity of the inducing current 

 is so feeble that it cannot traverse any sensible striking distance : 

 but it was also shown, that the more intense the electricity the less 

 the sum of force transmitted in a given time by the action of the 

 same battery and apparatus ; and that when the interruption of the 

 secondary circuit was the smallest possible, as by a hair's-breadth, 

 the largest amount of electricity passed through the galvanometer 

 connected with it. The power of the induced current to pass 

 through six inches or more of rarified air, was shown in the form of 

 Gassiot's cascade :* and the conversion of the dynamic force of the 

 primary and inducing current into the static force of the induced 

 electricity, was illustrated by the charging of electrometers and 

 Leyden jars. 



When the secondary current is interrupted, as just described, the 

 inducing power of the primary current acts in its own wire to pro- 



* Phil. Mag. 1854, vii. p. 99. 



