On Electro-Dynamic Induction. 149 



galvanometer would be produced. If these inductions are not 

 precisely equal, then, only a partial neutralization will take place, 

 and the shock will only be diminished in power ; and, also, per- 

 haps, the needle will be very slightly affected. 



84. If, in the foregoing exposition, we throw out of considera- 

 tion the actions of the feeble currents which cannot pass the 

 body, and, consequently, are not concerned in producing the 

 shock, then the same explanation will still apply which was 

 given in the last paper, (III, 94,) namely, in the above example, 

 the helix is acted on by the minus influence of the secondary, 

 and the plus influence of the interposed current. 



85. We are now prepared to consider the eff'ect on the helix 

 (Fig. 3) of the induced currents produced in the conductor of the 

 primary current itself. These are true secondary currents, and 

 are almost precisely the same in their action as those in the inter- 

 posed plate. Let us first examine the induced current at the be- 

 ginning of the primary, in the case of a long coil and a battery 

 of a single element ; its action on the helix may be represented 

 by the parts of the curve, Fig. 20. The first part, AB, will pro- 

 duce an intense induction opposite to that of the primary current ; 

 and hence the action of the two will tend to neutralize each 

 other, and no shock, or a very feeble one, will be produced. The 

 ending action of the same induced current, which is represented 

 by BD, restores to the helix the same quantity of current elec- 

 tricity (but in a feeble state) which was neutralized by AB, and 

 hence the needle of the galvanometer will be as much affected 

 as if this current did not exist. These inferences perfectly agree 

 with the experiment given in paragraph 19. In this, when the 

 ends of the interposed coil were joined so as to neutralize the 

 induced current in the long conductor, the shock at the beginning 

 of the primary current was nearly as powerful as with a short 

 conductor, while the amount of deflection of the galvanometer 

 was unaffected by joining the ends of the same coil. 



86. At first sight it might appear that any change in the appa- 

 ratus which might tend to increase the induction of the primary 

 current (16) would also tend to increase, in the same degree, the 

 adverse secondary in the same conductor ; and that hence the 

 neutralization mentioned in the last paragraph would take place 

 in all cases ; but we must recollect that if a more full current 

 be suddenly formed in a conductor of a given thickness, the ad- 



