zinc and copper plates, separated everywhere else by the acid used to
excite the combination, or by air. The bright spark and the shock, produced
only on breaking contact, are therefore effects of the same kind as those
produced in a higher degree by the helix, and in a still higher degree by
the electro-magnet.

1065. In order to compare an extended wire with a helix, the helix i,
containing ninety-six feet, and ninety-six feet of the same-sized wire
lying on the floor of the laboratory, were used alternately as conductors:
the former gave a much brighter spark at the moment of disjunction than the
latter. Again, twenty-eight feet of copper wire were made up into a helix,
and being used gave a good spark on disjunction at the electromotor; being
then suddenly pulled out and again employed, it gave a much smaller spark
than before, although nothing but its spiral arrangement had been changed.

1066. As the superiority of a helix over a wire is important to the
philosophy of the effect, I took particular pains to ascertain the fact
with certainty. A wire of copper sixty-seven feet long was bent in the
middle so as to form a double termination which could be communicated with
the electromotor; one of the halves of this wire was made into a helix and
the other remained in its extended condition. When these were used
alternately as the connecting wire, the helix half gave by much the
strongest spark. It even gave a stronger spark than when it and the
extended wire were used conjointly as a double conductor.

1067. When a _short wire_ is used, _all_ these effects disappear. If it be
only two or three inches long, a spark can scarcely be perceived on
breaking the junction. If it be ten or twelve inches long and moderately
thick, a small spark may be more easily obtained. As the length is
increased, the spark becomes proportionately brighter, until from extreme
length the resistance offered by the metal as a conductor begins to
interfere with the principal result.

1068. The effect of elongation was well shown thus: 114 feet of copper
wire, one-eighteenth of an inch in diameter, were extended on the floor and
used as a conductor; it remained cold, but gave a bright spark on breaking
contact. Being crossed so that the two terminations were in contact near
the extremities, it was again used as a conductor, only twelve inches now
being included in the circuit: the wire became very hot from the greater
quantity of electricity pa