On Electro- Dynamic Induction. 145 



of the twenty feet of the primary current would severally pro- 

 duce an inductive action of the same intensity as that of the 

 short conductor, the velocity of collapsion being the same ; and 

 as they are all at once exerted on the same conductor, a secon- 

 dary current would result of twenty times the intensity of the 

 current in the former case. 



77. To render this explanation more explicit, it may be proper 

 to mention that a current produced by an induction on one part 

 of a long conductor of uniform diameter, must exist, of the same 

 intensity, in every other part of the conductor ; hence, the action 

 of the several units of length of the primary current must enforce 

 each other, and produce the same effect on its own conductor 

 that the same current would if it were in a coil, and acting on a 

 helix. I need scarcely add, that in this case, as in that given in 

 paragraph 74, the whole amount of induction is greater with the 

 long conductor than with the short one, because the quantity of 

 current electricity is greater in the former than in the latter. 



78. We may next consider the character of the secondary cur- 

 rent, in reference to its action in producing a tertiary current in a 

 third conductor. The secondary current consists, as we may 

 suppose, in the disturbance, for an instant, of the natural electri- 

 city of the metal, which, subsiding, leaves the conductor again 

 in its natural state ; and whether it is produced by the beginning 

 or ending of a primary current, its nature, as we have seen, (22,) 

 is the same. Although the time of continuance of the secondary 

 current is very short, still we must suppose it to have some dura- 

 tion, and that it increases, by degrees, to a state of maximum 

 development, and then diminishes to the normal condition of the 

 metal of the conductor ; the velocity of its development, like 

 that of the primary current, will depend on the intensity of the 

 action by which it is generated, and also, perhaps, in some de- 

 gree, on the resistance of the conductor ; while, agreeably to the 

 hypothesis we have assumed, (69,) the velocity of its diminution 

 is nearly a constant quantity, and is not affected by changes in 

 these conditions ; hence, if we suppose the induction which pro- 

 duces the secondary current to be sufficiently intense, the velo- 

 city of its development will exceed that of its diminution, as in 

 the example of the primary current from the intense source of 

 the compound battery of many elements. Now this is the case 

 with the inductions which produce currents of the different or- 



Vol. xLi, No. 1.— April-June, 1&41. 19 



