On Electro- Dynamic Induction. 147 



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 BC being too feeble in intensity to produce a ter- 

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

 come the coercive power of the hardened steel. Hence, in refer- 

 ence to the shock, and magnetization of the steel needle, we may 

 entirely neglect the action of BC, and consider the tertiary ex- 

 citement as a single current, produced by the action of AB ; 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 opposite to that 

 of the current which produced it, and so on : we have here, 

 therefore, a simple explanation of the extraordinary phenomenon 

 of the alternation of the directions of the currents, of the differ- 

 ent orders, as given in this (25) and the preceding paper. 



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, (III, 64,) and that 

 this must react on the secondary, in the helix ; but it should not 

 alter the total amount of this current, since, for example, at the 

 ending induction, the same quantity of current is added to the 

 helix, while the current in the plate is decreasing, as is subtracted 

 while the same current is increasing. To make this more clear, 

 let the inductive actions of the interposed current be represented 

 by the parts of the curve. Fig. 20. The induction represented 

 by AB will react on the current in the helix, and diminish its 

 quantity, by an amount represented by the ordinate iB ; but the 

 induction represented by BC, will act in the next moment, on 

 the same current, and increase its quantity by an equal amount, 

 as represented by the same ordinate B6; and since both actions 

 take place within a small part of the time of a single swing of 

 the needle, the whole deflection will not be altered, and conse- 

 quently, as far as the galvanometer is concerned, the interposition 

 of the plate will have no perceptible effect." 



