ion; and by parity of reasoning, a similar
effect should take place when the conducting wire is also that in which the
induced current is formed (74.): hence the reason why a long wire gives a
brighter spark on breaking contact than a short one (1068.), although it
carries much less electricity.

1094. If the long wire be made into a helix, it will then be still more
effective in producing sparks and shocks on breaking contact; for by the
mutual inductive action of the convolutions each aids its neighbour, and
will be aided in turn, and the sum of effect will be very greatly
increased.

1095. If an electro-magnet be employed, the effect will be still more
highly exalted; because the iron, magnetized by the power of the continuing
current, will lose its magnetism at the moment the current ceases to pass,
and in so doing will tend to produce an electric current in the wire around
it (37. 38.), in conformity with that which the cessation of current in the
helix itself also tends to produce.

1096. By applying the laws of the induction of electric currents formerly
developed (6. &c.), various new conditions of the experiments could be
devised, which by their results should serve as tests of the accuracy of
the view just given. Thus, if a long wire be doubled, so that the current
in the two halves shall have opposite actions, it ought not to give a
sensible spark at the moment of disjunction: and this proved to be the
case, for a wire forty feet long, covered with silk, being doubled and tied
closely together to within four inches of the extremities, when used in
that state, gave scarcely a perceptible spark; but being opened out and the
parts separated, it gave a very good one. The two helices i and ii being
joined at their similar ends, and then used at their other extremities to
connect the plates of the electromotor, thus constituted one long helix, of
which one half was opposed in direction to the other half: under these
circumstances it gave scarcely a sensible spark, even when the soft iron
core was within, although containing nearly two hundred feet of wire. When
it was made into one consistent helix of the same length of wire it gave a
very bright spark.

1097. Similar proofs can be drawn from the mutual inductive action of two
separate currents (1110.); and it is important for the general principles
that the consistent action of two such currents should be established.
Thus, two currents going in the same direct