148 Contributions to Electricity and Magnetism. 
82. ‘But the effect of the plate on the shock, and on the mag- 
netization of tempered 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 in- 
terposed current, as to fall below that degree which enables it to 
penetrate the body, or overcome the coercive force of the steel. 
To understand 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 mo- 
mentary current, in the direction of the primary 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 AB, and BC, of the curve, Fig. 20; the 
first of these, AB, 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, (BC,) 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 he- 
lix will not be able to penetrate the body; no shock will be per- 
ceived, or at least a very slight one, and the phenomena of 
screening will be exhibited. 
83, When the plate of metal is placed between the conductors 
of the second and third orders, or between those of the third and 
fourth, the action is somewhat different, although the general 
principle is the same. Let us suppose the plate interposed be- 
tween the second and third conductors; then the helix, or third 
conductor, will be acted on by four inductions, two from the sec- 
ondary current, and two from the current in the plate. The di- 
rection and character of these will be as follows, on the supposi- 
tion that the direction of the secondary current is itself plus : 
The beginning secondary . intense and. . minus. 
The ending secondary . . feebleand . . plus. 
The beginning interposed . intense and. . plus. 
The ending interposed . . feebleand . . minus. 
Now if the action, on the third conductor, of the first and third 
of the above inductions be equal in intensity and quantity, they 
will neutralize each other; and the same will also take place 
with the action of the second and fourth, if they be equal, and 
hence, in this case, neither shock hor motion of the needle of the 
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