heat, at
the opposite side of the world.

Thus, the light and heat produced outside the battery are derived from
the metallic fuel burnt within the battery; and, as zinc happens to be
an expensive fuel, though we have possessed the electric light for
more than seventy years, it has been too costly to come into general
use. But within these walls, in the autumn of 1831, Faraday
discovered a new source of electricity, which we have now to
investigate. On the table before me lies a coil of covered copper
wire, with its ends disunited. I lift one side of the coil from the
table, and in doing so exert the muscular effort necessary to overcome
the simple weight of the coil. I unite its two ends and repeat the
experiment. The effort now required, if accurately measured, would be
found greater than before. In lifting the coil I cut the lines of the
earth's magnetic force, such cutting, as proved by Faraday, being
always accompanied, in a closed conductor, by the production of an
'induced' electric current which, as long as the ends of the coil
remained separate, had no circuit through which it could pass. The
current here evoked subsides immediately as heat; this heat being the
exact equivalent of the excess of effort just referred to as over and
above that necessary to overcome the simple weight of the coil. When
the coil is liberated it falls back to the table, and when its ends
are united it encounters a resistance over and above that of the air.
It generates an electric current opposed in direction to the first,
and reaches the table with a diminished shock. The amount of the
diminution is accurately represented by the warmth which the momentary
current developer in the coil. Various devices were employed to exalt
these induced currents, among which the instruments of Pixii, Clarke,
and Saxton were long conspicuous. Faraday, indeed, foresaw that such
attempts were sure to be made; but he chose to leave them in the hands
of the mechanician, while he himself pursued the deeper study of facts
and principles. 'I have rather,' he writes in 1831, 'been desirous
of discovering new facts and new relations dependent on
magneto-electric induction, than of exalting the force of those
already obtained; being assured that the latter would find their full
development hereafter.'

For more than twenty years magneto-electricity had subserved its first
and noblest purpose of augmenting our knowledge of the powers of
nature.