1840] WRITINGS OF JOSEPH HENRY. 183 



in the plate is decreasing, as is subtracted while the same 

 current is increasing. To make this more clear, let the in- 

 ductive actions of the interposed current be represented by 

 the parts of the curve, Fig. 20. The induction represented 

 by A B will re-act on the current in the helix, and diminish 

 its quantity, by an amount represented by the ordinate b B; 

 but the induction represented by B C, will act in the next 

 moment, on the same current, and increase its quantity by 

 an equal amount, as represented by the same ordinate B b; 

 and since both actions take place within a small part of the 

 time of a single swing of the needle, the whole deflection 

 will not be altered, and consequently, as far as the galva- 

 nometer is concerned, the interposition of the plate will have 

 no perceptible effect. 



82. But the effect of the plate on the shock, and on the 

 magnetization 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 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 i^ro- 

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

 in the direction of the primary current, which we have called 

 ■plus; the current in the plate will re-act on the helix, and 

 tend to produce in it two inductions, which as before may 

 be represented 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 sec- 

 ond, {B 0,) will add to the helix an equal quantity of in- 

 duced current, but of a much more feeble intensity, and 

 hence the resulting current 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 phenomena of screening will be 

 exhibited. 



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



