512 Prof. J. Henry's Contributions 



diminish its effect. Also, b}' diminishing the length of the 

 primary current, we must diminish (76.) the intensity of the 

 secondary, so that it will meet with more resistance in passing 

 the acid of the single battery, and thus its effects be dimi- 

 nished. 



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 perceived 

 that its action in this respect must be much more feeble than 

 that of the similar current at the beginning; the latter is pro- 

 duced at the moment of making contact, and hence it is pro- 

 pagated in a continuous circuit of conducting matter, while 

 the other takes place at the rupture of the circuit, and must 

 therefore be rendered comparatively feeble by being obliged 

 to pass through a small portion of heated air; very little effect 

 is therefore produced on the helix by this induction (19.). 

 The fact that this current 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 explained, since, in this case, the body forms, with the 

 conductor, 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 

 betjinninj; and endinfj 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 contact is 

 made and broken with mercury. We may suppose, from the 

 existence of analogous phaenomena 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 in- 

 tensity, or sunk more nearly to its normal state. These varia- 

 tions are indicated by the rapid rise of the curve, fig. 17, from 

 A to^, and the more gradual increase of the ordinates from // 

 to B; and by the rapid diminution of the ordinates between 

 C and /, and the gradual decrease of those towards the end of 

 the curve. 



89. These more minute considerations, relative to the form 

 of the curve, will enable us to conceive how the time of the 

 ending of the secondary current, as we have suggested (78.), 

 may be prolonged beyond that of the natural subsidence of 



