ON ELECTRO-DYNAMIC INDUCTION. 23 



to the same conclusion, (18,) (90,) and also to the farther inference that the decline 

 of the current is not instantaneous. According to this view, therefore, the induc- 

 tive actions at the beginning and the ending of a primary current, of which the 

 formation and interruption is effected by means of the contact with a cup of 

 mercury, may also be represented by the several parts of the curve, Fig. 17. 



68. We have now to consider how the rate of increase or diminution of the 

 current, in the case in question, can be altered by a change in the different 

 parts of the apparatus ; and, first, let us take the example of a single battery 

 and a short conductor, making only one or two turns around the helix; with 

 this arrangement a feeble shock, as we have seen, (11,) will be felt at the 

 making, and also at the breaking of the circuit. In this case it would seem 

 that almost the only impediment to the most rapid development of the current 

 would be the resistance to conduction of the metal ; and this we might sup- 

 pose would be more rapidly overcome by increasing the tension of the electri- 

 city; and, accordingly, we find that if the number of elements of the battery be 

 increased, the shock at making the circuit will also be increased, while that at 

 breaking the circuit will remain nearly the same. To explain, however, this 

 effect more minutely, we must call to mind the fact before referred to, (17,) 

 that when the poles of a compound battery are not connected, the apparatus 

 acquires an accumulation of electricity, which is discharged at the first mo- 

 ment of contact, and which, in this case, would more rapidly dev elope the full 

 current, and hence produce the more intense action on the helix at making the 

 circuit. 



69. The shock, and also the deflection of the needle, at breaking the circuit 

 with a compound battery and a short coil, (9,) appears nearly the same as with 

 a battery of a single element, because the accumulation just mentioned, in the 

 compound battery, is discharged almost instantly, and, according to the theory 

 (71) of the galvanic current, leaves the constant current in the conductor nearly 

 in the same state of quantity as that which would be produced by a battery of 

 a single element; and hence the conditions of the ending of the current are the 

 same in both cases. Indeed, in reference to the ending induction, it may be 

 assumed as a fact which is in accordance with all the experiments, (9, 13, 73, 

 74, 75, 76, &c.,) as well as with theoretical considerations,* that when the cir^ 



* See the theory of Ohm. 



