ON ELECTRO-DYNAMIC INDUCTION. 7 



of increasing, at pleasure, the intensity of the induction at the beginning of a 

 battery current, the one consisting in increasing the intensity of the source of 

 the electricity, and the other in diminishing the resistance to conduction of the 

 circuit while its intensity remains the same. 



17. The explanation of the effects which we have given, relative to the in- 

 duction at the beginning, is apparently not difficult. The resistance to con- 

 duction in the case of a long conductor and a battery of a single element is so 

 great that the full development of the primary current may be supposed not to 

 take place with sufficient rapidity to produce the instantaneous action on which 

 the shock from the secondary current would seem to depend. But when a 

 battery of a number of elements is employed, the poles of this, previous to the 

 moment of completing the circuit, are in a state of electrical tension; and there- 

 fore the discharge through the conductor may be supposed to be more sudden, 

 and hence an induction of more intensity is produced. 



18. That the shock at both making and breaking the circuit in some way 

 depends on the rapidity of formation and diminution of the current is shown 

 by the following experiment, in which the tension just mentioned does not take 

 place, and in which, also, the current appears to diminish more slowly. The 

 two ends of the coil were placed in the two cups which formed the poles 

 of the battery, and permanently retained there during the experiment; also, at 

 the distance of about six inches from, say the right hand end of the coil, a loop 

 was made in the riband, which could be plunged into the cup containing the 

 left hand end. With this arrangement, and while only the two extreme ends 

 of the coil were in connexion with the cups of mercury, of course the current 

 passed through the entire length of the riband of the coil, but by plunging the 

 loop into the left hand cup, the whole length of the coil, except the six inches 

 before mentioned, was excluded from the battery circuit. And again, when 

 the loop was lifted out of the cup, the whole length was included. In this way 

 the current in the coil could be suddenly formed and interrupted, while the 

 poles of the battery were continually joined by a conductor, but no shock with 

 either a single or a compound battery could be obtained by this method of 

 operation. 



19. The feebleness of the shock at the beginning of the current, with a sin- 

 gle battery and a long coil, is not entirely owing to the cause we have stated, 

 (17,) namely, the resistance to conduction offered by the long conductor, but 



