184 WRITINGS OF JOSEPH HENRY. [1840 



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

 the third and fourth, the action is somewhat different, al- 

 though the general principle is the same. Let us suppose 

 the plate interposed between the second and third conduc- 

 tors; 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 direc- 

 tion 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, it 

 they be equal, and hence in this case, neither shock nor 

 motion of the needle of the galvanometer would be pro- 

 duced. If these inductions be not precisely equal, then 

 only a partial neutralization will take place, and the shock 

 will be merely diminished in power; and also the needle" 

 will perhaps be very slightly affected. 



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

 sideration the actions of the feeble currents which cannot 

 pass the body, and which consequently are not concerned 

 in producing the shock, then the same explanation will 

 still apply which was given in the last paper, (No. Ill, 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 con- 

 ductor of the primary current itself These are true second- 

 ary currents, and are almost precisely the same in their ac- 

 tion as those in the interposed plate. Let us first examine 

 the induced currents at the beginning of the primary, in 

 the case of a long coil and a battery of a single element. 



