186 WRITINGS OF JOSEPH HENRY. [1840 



87. The action of the secondary current in the long coil 

 at the ending of the primary current, should also at first 

 sight produce the same screening influence as the current 

 in the interposed plate; but on reflection it will be per- 

 ceived that its action in this respect must be much more 

 feeble that that of the similar current at the beginning; 

 the latter is produced at the moment of making contact, 

 and hence it is propagated in a continuous circuit of con- 

 ducting matter, while the other takes place at the rupture 

 of the circuit, and must therefore be rendered compara- 

 tively feeble by being obliged to pass through a small por- 

 tion of heated air ; very little effect is therefore produced 

 on the helix by this induction, (19.) The fact that this cur- 

 rent is capable of giving intense shocks, when the ends of a 

 long wire which is transmitting a primary current, are 

 grasped at the time of breaking the circuit, is readily ex- 

 plained, since in this case the body forms with the con- 

 ductor a closed circuit, which permits the comparatively 

 free circulation of the induced current. 



88. It will be seen that I have given a peculiar form to the 

 beginning and ending of the curves. Figs. 17, 18, &c. These 

 are intended to represent the variations which may be sup- 

 posed to take place in the rate of increase and decrease of 

 the quantity of the current, even in the case where the con- 

 tact is made and broken with mercury. We may suppose, 

 from the existence of analogous phenomena in magnetism, 

 heat, &c., that the development of the current would be more 

 rapid at first than when it approximates what may be called 

 the state of current saturation, or when the current has 

 reached more nearly the limit of capacity of conduction of 

 the metal. Also, the decline of the current may be supposed 

 to be more rapid at the first moment, than after it has lost 

 somewhat of its intensity, or sunk more nearly to its normal 

 state. These variations are indicated by the rapid rise of the 

 curve. Fig. 17, from A to g, and the more gradual increase 

 of the ordinates from h to B ; and by the rapid diminution 

 of the ordinates between C and I, and the gradual decrease 

 of those towards the end of the curve. 



