It will be seen by reference to these results that the percentage of
inductive energy intercepted does not increase for different speeds of
the reverser in the same rate with different metals, the increase with
iron being very slight, while with tin it is comparatively enormous. It
was observed that time was an important element to be taken into account
while testing the above metals, that is to say, the lines of force took
an appreciable time to polarize the particles of the metal placed in
their path, but having accomplished this, they passed more freely
through it.

Now let us go more minutely into the subject by the aid of Plate IV.,
Figs. 1 and 2. In Fig. 1 let A and B represent two flat spirals, spiral
A being connected to a battery with a key in circuit and spiral B
connected to a galvanometer; then, on closing the battery circuit, an
instantaneous current is induced in spiral B. If a non-magnetic metal
plate half an inch thick be placed midway between the spirals, and the
experiment repeated, it will be found that the induced current received
by B is the same in amount as in the first case. This does not prove,
as would at first appear, that the metal plate fails to intercept the
inductive radiant energy; and it can scarcely be so, for if the plate is
replaced by a coil of wire, it is found that induced currents are set
up therein, and therefore inductive radiant energy must have been
intercepted. This apparent contradiction may be explained as follows:

In Fig. 2 let D represent a source of heat (a vessel of boiling water
for instance) and E a sensitive thermometer receiving and measuring the
radiant heat. Now, if for instance a plate of vulcanite is interposed,
it cuts off and absorbs a part of the radiant heat emitted by D, and
thus a fall is produced in the thermometer reading. But the vulcanite,