scribed the current excited in iron could not be
transmitted but through the copper, and that excited in copper had to pass
through iron: i.e. supposing currents of dissimilar strength to be formed
in the metals proportionate to their conducting power, the stronger current
had to pass through the worst conductor, and the weaker current through the
best.

204. Experiments were therefore made in which different metals insulated
from each other were passed between the poles of the magnet, their opposite
ends being connected with the same end of the galvanometer wire, so that
the currents formed and led away to the galvanometer should oppose each
other; and when considerable lengths of different wires were used, feeble
deflections were obtained.

205. To obtain perfectly satisfactory results a new galvanometer was
constructed, consisting of two independent coils, each containing eighteen
feet of silked copper wire. These coils were exactly alike in shape and
number of turns, and were fixed side by side with a small interval between
them, in which a double needle could be hung by a fibre of silk exactly as
in the former instrument (87.). The coils may be distinguished by the
letters KL, and when electrical currents were sent through them in the same
direction, acted upon the needle with the sum of their powers; when in
opposite directions, with the difference of their powers.

206. The compound helix (199. 8.) was now connected, the ends A and B of
the iron with A and B ends of galvanometer coil K, and the ends A and B of
the copper with B and A ends of galvanometer coil L, so that the currents
excited in the two helices should pass in opposite directions through the
coils K and L. On introducing a small cylinder magnet within the helices,
the galvanometer needle was powerfully deflected. On disuniting the iron
helix, the magnet caused with the copper helix alone still stronger
deflection in the same direction. On reuniting the iron helix, and
unconnecting the copper helix, the magnet caused a moderate deflection in
the contrary direction. Thus it was evident that the electric current
induced by a magnet in a copper wire was far more powerful than the current
induced by the same magnet in an equal iron wire.

207. To prevent any error that might arise from the greater influence, from
vicinity or other circumstances, of one coil on the needle beyond that of
the other, the iron and copper terminations were changed relative t