510 Prof. J. Henry's Contributiotis 



foi' although the quantity of induction in the helix may not 

 be changed, yet its intensity may be so reduced, by the ad- 

 verse 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 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 pro- 

 duce, in both the helix and the plate, a momentary current, 

 in the direction of the j)rimary current, which we have called 

 phis; the current in the plate will react on the helix, and tend 

 to produce in it two inductions, which, as before, may be re- 

 presented 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 (B C) 

 will add to the helix an equal quantity of induced current, but 

 of a much more feeble intensity, and hence the resulting cur- 

 rent in the helix will not be able to penetrate the body; no 

 shock will be perceived, or at least a very slight one, and the 

 phaenomena of screening v/ill be exhibited. 



83. When the plate of metal is placed between the con- 

 ductors of the second and third orders, or between those of 

 the third and fourth, the action is somewhat diffiarent, although 

 the general principle is the same. Let us suppose the plate 

 interposed between the second and third conductors ; then the 

 helix, or third conductor, will be acted on by four inductions, 

 two from the secondary current and two from the current in 

 the plate. The direction and character of these will be as 

 follows, on the supposition that the direction of the secondary 

 current is itself phis : 



The beginning secondary . . . intense and . . . mifius. 



The ending secondary feeble and .... phis. 



The beginning interposed . . . intense and . . . phis. 

 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 quan- 

 tity, 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 galvanometer would be produced. If these 

 inductions are not precisely equal, then only a partial neutra- 

 lization will take place, and the shock will only be diminished 

 in power; and, also, perhaps, the needle will be very slightly 

 affected. 



Sir. If, in the foregoing exposition, we throw out of con- 



