itself has a perceptible duration depending upon the nature and mass of
metal thus interposed. Copper and zinc produce in this manner an induced
current of greater length than metals of lower conductivity, with the
exception of iron, which gives an induced current of extremely short
duration. It will therefore be seen that in endeavoring to ascertain
what I term the specific inductive resistance of different metals by
the means described, notice must be taken of and allowance made for
two points. First, that the metal plate not only cuts off, but itself
radiates; and secondly, that the duration of the induced currents
radiated by the plates varies with each different metal under
experiment.

This explains the fact before pointed out that the apparent percentage
of inductive radiant energy intercepted by metal plates varies with the
speed of the reversals; for in the case of copper the induced current
set up by such a plate has so long a duration that if the speed of the
reverser is at all rapid the induced current has not time to exhaust
itself before the galvanometer is reversed, and thus the current being
on the opposite side of the galvanometer tends to produce a lower
deflection. If the speed of the reverser be further increased, the
greater part of the induced current is received on the opposite terminal
of the galvanometer, so that a negative result is obtained.

We know that it was the strong analogies which exist between electricity
and magnetism that led experimentalists to seek for proofs that would
identify them as one and the same thing, and it was the result of
Professor Oersted's experiment to which I have already referred that
first identified them.

Probably the time is not far distant when it will be possible to