With a view to proving this experimentally, Faraday constructed three
flat helices and placed them parallel to each other a convenient
distance apart. The middle helix was so arranged that a voltaic current
could be sent through it at pleasure. A differential galvanometer was
connected with the other helices in such a manner that when a voltaic
current was sent through the middle helix its inductive action on
the lateral helices should cause currents in them, having contrary
directions in the coils of the galvanometer. This was a very prettily
arranged electric balance, and by placing plates of different substances
between the inductor and one of the inductometers Faraday expected to
see the balance destroyed to an extent which would be indicated by the
deflection of the needle of the galvanometer. To his surprise he found
that it made not the least difference whether the intervening space was
occupied by such insulating bodies as air, sulphur, and shellac, or such
conducting bodies as copper and the other non-magnetic metals. These
results, however, did not satisfy him, as he was convinced that the
interposition of the non-magnetic metals, especially of copper, did
have an effect, but that his apparatus was not suitable for making it
visible. It is to be regretted that so sound a reasoner and so careful
an experimenter had not the great advantage of the assistance of
such suitable instruments for this class of research as the
mirror-galvanometer and the telephone. But, although he could not
practically demonstrate the effects which by him could be so clearly
seen, it redounds to his credit that, as the improvement in instruments
for this kind of research has advanced, the results he sought for have
been found in the direction in which he predicted.

A and B will now be placed a definite distance apart, and comparatively
slow reversals from ten Leclanche cells sent through spiral A; you will
observe the amount of the induced current in B, as shown on the scale of