ON ELECTRO-Df NAMIC INDUCTION. 29 



79. It will be seen at once, by an inspection of the curve, that the effect 

 produced, in a third conductor, and which we have called a tertiary current, 

 is not of the same nature as that of a secondary current. Instead of being a single 

 development in one direction, it consists of two instantaneous currents, one pro- 

 duced by the induction of A B, and the other, by that of B C, m opposite di- 

 rections, of equal quantities, but of different intensities. The whole quantity 

 of induction in the two directions, will each be represented by the ordinate 

 B b, and hence they will nearly neutralize each other, in reference to their 

 action on the galvanometer, in the circuit of the third conductor. I say, they 

 will nearly neutralize each other, because, although they are equal in quantity, 

 they do not both act in absolutely the same moment of time. The needle will, 

 therefore, be slightly affected; it will be impelled in one direction, say to the 

 right, by the induction of A B, but, before it can get fairly under w-ay, it will 

 be arrested, and turned in the other direction, by the action of B C. This 

 inference is in strict accordance with observation; the needle, as we have seen, 

 (24) starts from a state of rest, with a velocity which, apparently, would send it 

 through a large arc, but before it has reached, perhaps, more than half a degree, 

 it suddenly stops, and turns in the other direction. As the needle is first af- 

 fected by the action of A B, it indicates a current in the adverse direction to 

 the secondary current. 



80. Although the two inductions in the tertiary conductor nearly neutralize 

 each other, in reference to the indications of the galvanometer, yet this is far 

 from being the case with regard to the shocks, and the magnetization of steel 

 needles. These effects may be considered as the results alone of the action 

 of AB; the induction of B C being too feeble in intensity to produce a ter- 

 tiary current of sufficient power to penetrate the body, or overcome the co- 

 ercive power of the hardened steel. Hence, in reference to the shock, and 

 magnetization of the steel needle, we may entirely neglect the action of B C, 

 and consider the tertiary excitement as a single current, produced by the action 

 oi AB; and, because this is the beginning induction, the tertiary current must 

 be in an opposite direction to the secondary. For a similar reason, a current of 

 the third order should produce in effect a single current of the fourth order, 

 in a direction opposite to that of the current which produced it, and so on: we 

 have here, therefore, a simple explanation of the extraordinary phenomenon 



VIII. — H 



