the effect at the galvanometer was very powerful.

199. The compound helix of copper and iron wire formerly described (8.) was
arranged as a double helix, one of the helices being all iron and
containing two hundred and fourteen feet, the other all copper and
continuing two hundred and eight feet. The two similar ends AA of the
copper and iron helix were connected together, and the other ends BB of
each helix connected with the galvanometer; so that when a magnet was
introduced into the centre of the arrangement, the induced currents in the
iron and copper would tend to proceed in contrary directions. Yet when a
magnet was inserted, or a soft iron bar within made a magnet by contact
with poles, no effect at the needle was produced.

200. A glass tube about fourteen inches long was filled with strong
sulphuric acid. Twelve inches of the end of a clean copper wire were bent
up into a bundle and inserted into the tube, so as to make good superficial
contact with the acid, and the rest of the wire passed along the outside of
the tube and away to the galvanometer. A wire similarly bent up at the
extremity was immersed in the other end of the sulphuric acid, and also
connected with the galvanometer, so that the acid and copper wire were in
the same parallel relation to each other in this experiment as iron and
copper were in the first (194). When this arrangement was passed in a
similar manner between the poles of the magnet, not the slightest effect at
the galvanometer could be perceived.

201. From these experiments it would appear, that when metals of different
kinds connected in one circuit are equally subject in every circumstance to
magneto-electric induction, they exhibit exactly equal powers with respect
to the currents which either are formed, or tend to form, in them. The same
even appears to be the case with regard to fluids, and probably all other
substances.

202. Still it seemed impossible that these results could indicate the
relative inductive power of the magnet upon the different metals; for that
the effect should be in some relation to the conducting power seemed a
necessary consequence (139.), and the influence of rotating plates upon
magnets had been found to bear a general relation to the conducting power
of the substance used.

203. In the experiments of rotation (81.), the electric current is excited
and discharged in the same substance, be it a good or bad conductor; but in
the experiments just de