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 j!)/ms ; 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 heUx; 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. 



63. 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 . . feeble and . . plus. 



The beginning interposed . intense and . . plus. 



The ending interposed . . feeble and . . 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 nor motion of the needle of the 



