ON ELECTRO-DYNAMIC INDUCTION. 31 



will not be able to penetrate the body ; no shock will be perceived, 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 some- 

 what different, 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 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 galvanometer would be produced. If these inductions are not precisely 

 equal, then, only a partial neutralization will take place, and the shock will only 

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

 affected. 



84. If, in the foregoing exposition, we throw out of consideration the actions 

 of the feeble currents which cannot pass the body, and, consequently, are not 

 concerned in producing the shock, then the same explanation will still apply 

 which was given in the last paper, (III., 94,) namely, in the above example, 

 the helix is acted on by the minus influence of the secondary, and the plus 

 influence of the interposed current. 



85. We are now prepared to consider the effect on the helix (Fig. 3) of the 

 induced currents produced in the conductor of the primary current itself. 

 These are true secondary currents, and are almost precisely the same in their 

 action as those in the interposed plate. Let us first examine the induced cur- 

 rent at the beginning of the primary, in the case of a long coil and a battery of 

 a single element; its action on the helix may be represented by the parts of the 



