On Electro-Dynamic Induction. 123 
ceived. We have, therefore, the remarkable result, that the in- 
tensity of the ending induction of each unit of length of the 
battery current is not materially altered, at least within certain 
limits, by changing the length of the whole conductor. From 
this we would infer that the shock depends more on the intensity 
of the action than on the quantity of the current, since we know 
that the latter is diminished in a given unit of the conductor by 
increasing the length of the whole. 
14, We have seen (8) that with a circuit composed of ten ele- 
ments of the compound battery and the coil No. 2, the shock, at 
the beginning of the current, was fully equal to that at the end- 
ing. It was, however, found that if, in this case, the length of 
the coil was increased, this shock was diminished; and we may 
State, asan inference from several experiments, that however 
great may be the intensity of the electricity from the battery, 
the shock at the beginning may be so reduced by a sufficient in- 
crease of the length of the primary circuit, as to be scarcely per- 
ceptible. 
15. It was also found that when the thickness of the coil 
was increased, the length and intensity of the circuit remaining 
the same, the shock at the beginning of the battery current, was 
somewhat increased. This result was produced by using a 
double coil; the electricity was made to pass through one strand, 
and immediately afterwards through both: ‘the shock from the 
helix in the latter case was apparently the greater. . 
16. By the foregoing results we are evidently furnished with 
two methods of increasing, at pleasure, the intensity of the in- 
duction at the beginning of a battery current, the one consist- 
ing in increasing the intensity of the source of the electricity, 
and the other in diminishing the resistance to conduction of the 
circuit while the intensity remains the same. F 
17. The explanation of the effects which we have given, rel- 
ative to the induction at the beginning, is apparently not difficult. 
The resistance to conduction in the case of a long conductor and 
a battery of a single element isso great that the full develop- 
ment 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 de- 
pend. But when a battery of a number of elements Is employed, 
_ the poles of this, previous to the moment of completing the cir- 
