30 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



of the alternation of the directions of the currents, of the different orders, as 

 given in this 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 inter- 

 posed current be represented by the parts of the curve. Fig 20. The induction 

 represented hj A B will react on the current in the helix, and diminish its 

 quantity, by an amount represented by the ordinate b B; but the induction 

 represented by B C, will act in the next moment, on the same current, and 

 increase its quantity by an equal amount, as represented by the same ordinate 

 Bb; 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 consequently, 

 as far as the galvanometer is concerned, the interposition of the plate will have 

 no perceptible effect. 



82. But the action of the plate on the shock, and on the magnetization of tem- 

 pered steel, should be very different; for, although the quantity of induction in the 

 helix may not be changed, yet its intensity may be so reduced, by the adverse 

 action of the interposed current, as to fall below that degree which enables it 

 to penetrate the body, or overcome the coercive force of the steel. To under- 

 stand how this may be, let us again refer, for example, to the induction 

 which takes place at the ending of a battery current: this will produce, 

 in both the helix and the plate, a momentary current, in the direction of the pri- 

 mary current, which we have called plus; the current in the plate will react 

 on the helix, and tend to produce in it two inductions, which, as before, may 

 be represented by A B, and B C, of the curve. Fig. 20; the first of these, A B, 

 will be an intense action, (78,) in the minus direction, and will, therefore, 

 tend to neutralize the intense action of the primary current on the helix; the 

 second, (jB C,) will add to the helix an equal quantity of induced current, but 

 of a much more feeble intensity, and hence the resulting current in the helix 



