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 suflicient 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. ‘T’o make this more clear, 
let the inductive actions of the interposed current be represented 
by the parts of the curve, Fig. 20. The induction represent 
by AB will react on the current in the helix, and diminish its 
quantity, by an amount represented by the ordinate 6B; 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 Bd ; 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. 
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