e two ends of the wire which surrounds the
armature are now disconnected. In turning the handle and causing the
armature to rotate, I simply overcome ordinary mechanical friction.
But the two ends of the armature coil can be united in a moment, and
when this is done I immediately experience a greatly increased
resistance to rotation. Something over and above the ordinary
friction of the machine is now to be overcome, and by the expenditure
of an additional amount of muscular force I am able to overcome it.
The excess of labour thus thrown upon my arm has its exact equivalent
in the electric currents generated, and the heat produced by their
subsidence in the coil of the armature. A portion of this heat may be
rendered visible by connecting the two ends of the coil with a thin
platinum wire. When the handle of the machine is rapidly turned the
wire glows, first with a red heat, then with a white heat, and finally
with the heat of fusion. The moment the wire melts, the circuit round
the armature is broken, an instant relief from the labour thrown upon
the arm being the consequence. Clearly realise the equivalent of the
heat here developed. During the period of turning the machine a
certain amount of combustible substance was oxidised or burnt in the
muscles of my arm. Had it done no external work, the matter consumed
would have produced a definite amount of heat. Now, the muscular heat
actually developed during the rotation of the machine fell short of
this definite amount, the missing heat being reproduced to the last
fraction in the glowing platinum wire and the other parts of the
machine. Here, then, the electric current intervenes between my
muscles and the generated heat, exactly as it did a moment ago between
the voltaic battery and its generated heat. The electric current is
to all intents and purposes a vehicle which transports the heat both
of muscle and battery to any distance from the hearth where the fuel
is consumed. Not only is the current a messenger, but it is also an
intensifier of magical power. The temperature of my arm is, in round
numbers, 100 deg. Fahr, and it is by the intensification of this heat that
one of the most refractory of metals, which requires a heat of 3,600 deg.
Fahr. to fuse it, has been reduced to the molten condition.

Zinc, as I have said, is a fuel far too expensive to permit of the
electric light produced by its combustion being used for the common
purposes of life,