to Electricity and Magnetism. 507 



ductor of the primary current itself, and the secondary current 

 which is produced is generated by the joint action of each unit 

 of the length of the primary current. Let us suppose, for il- 

 lustration, that the conductor was at first one foot long, and 

 afterwards increased to twenty feet. In the first case, because 

 the short conductor would transmit a greater quantity of elec- 

 tricity, the secondary current produced by it would be one of 

 considerable quantity or power to deflect a galvanometer ; but 

 it would be of feeble intensity, for although the primary cur- 

 rent would collapse with its usual velocity (69.), yet, acting on 

 only a footof conducting matter, the elFect(74.) would befeeble. 

 In the second case, each foot of the twenty feet of the primary 

 current would severallyproduce an inductive action of the same 

 intensity as that of the short conductor, the velocity of collap- 

 sion being the same ; and as they are all at once exerted on the 

 same conductor, a secondary 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 con- 

 ductor; 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 elec- 

 tricity is greater in the former than in the latter. 



78. We may next consider the character of the secondary 

 current, in reference to its action in producing a tertiary cur- 

 rent in a third conductor. The secondary current consists, 

 as we may suppose, in the disturbance, for an instant, of the 

 natural electricity of the metal, which, subsiding, leaves the 

 conductor again in its natural state; and whether it is pro- 

 duced 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 duration, and that it increases, 

 by degrees, to a state of maximum development, and then di- 

 minishes to the normal condition of the metal of the conduc- 

 tor ; 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 degree, on the resist- 

 ance of the conductor; while, agreeably to the hypothesis we 

 have assumed (69. )> the velocity of its diminution is nearly a 



