elix,
corresponding to the two former, the three being parallel to each other.
The middle helix was so arranged that a voltaic current could be sent
through it at pleasure. The former galvanometer was removed, and one with a
double coil employed, one of the lateral helices being connected with one
coil, and the other helix with the other coil, in such manner that when a
voltaic current was sent through the middle helix its inductive action
(26.) on the lateral helices should cause currents in them, having contrary
directions in the coils of the galvanometer. By a little adjustment of the
distances these induced currents were rendered exactly equal, and the
galvanometer needle remained stationary notwithstanding their frequent
production in the instrument. I will call the middle coil C, and the
external coils A and B.

1721. A plate of copper 0.7 of an inch thick and six inches square, was
placed between coils C and B, their respective distances remaining
unchanged; and then a voltaic current from twenty pairs of 4 inch plates
was sent through the coil C, and intermitted, in periods fitted to produce
an effect on the galvanometer (1712.). if any difference had been produced
in the effect of C on A and B. But notwithstanding the presence of air in
one interval and copper in the other, the inductive effect was exactly
alike on the two coils, and as if air had occupied both intervals. So that
notwithstanding the facility with which any induced currents might form in
the thick copper plate, the coil outside of it was just as much affected by
the central helix C as if no such conductor as the copper had been there
(65.).

1722. Then, for the copper plate was substituted one of sulphur 0.9 of an
inch thick; still the results were exactly the same, i.e. there was no
action at the galvanometer.

1723. Thus it appears that when a voltaic current in one wire is exerting
its inductive action to produce a contrary or a similar current in a
neighbouring wire, according as the primary current is commencing or
ceasing, it makes not the least difference whether the intervening space is
occupied by such insulating bodies as air, sulphur and shell-lac, or such
conducting bodies as copper, and the other non-magnetic metals.

1724. A correspondent effect was obtained with the like forces when
resident in a magnet thus. A single flat helix (1718.) was connected with a
galvanometer, and a magnetic pole placed near to it; then by moving the