182 WRITINGS OF JOSEPH HENRY. [1840 



way, 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 affected 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 

 nearl}' 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 A B ; the induction of B C being too feeble in in- 

 tensity to produce a tertiary current of sufficient power to 

 penetrate the body, or overcome the coercive power of the 

 hardened steel. Hence, in reference to the shock, and mag- 

 netization of the steel needle, we maj'' entirely neglect the 

 action of B C, and consider the tertiary excitement as a 

 single current, produced by the action A B ; and because 

 this is the beginning induction, (56,) 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 op- 

 posite to that of the current which produced it, and so on ; 

 we have here therefore a simple explanation of the extraordi- 

 nary phenomenon of the alternation of the directions of the 

 currents of the different orders, as given in this and the pre- 

 ceding paper. (See paragraph 25.) 



81. The operation of the interposed plate, (32, 47, 48, &c.,) 

 in neutralizing the shock, and not affecting the galvanometer, 

 can also be readily referred to the same principles. It is 

 certain, that an induced current is produced in the plate (No. 

 Ill, 64,) and that this must re-act on the secondary, in the 

 helix ; but it should not alter the total amount of this cur- 

 rent, since for example at the ending induction, the same 

 quantity of current is added to the helix while the current 



