First,
the lamp glows, in a moment it again becomes dark, then, as the dynamo
gets up speed, glows again. If the brake be screwed up tight, the lamp
once more becomes dark. The explanation is simple. Owing to the
coefficient of self-induction of the dynamo machine being
considerable, it takes a finite time for the current to obtain an
appreciable intensity, but the lamp having no self-induction, the
current at once passes through it, and causes it to glow. Secondly,
the electrical inertia of the dynamo being overcome, it must draw a
large current to produce the kinetic energy of rotation, i.e., to
overcome its mechanical inertia; the lamp is therefore practically
short-circuited, and ceases to glow. When once the rotation has been
established, the current through the dynamo becomes very small, having
no work to do except to overcome the friction of the bearings, hence
the lamp again glows. Finally, by screwing up the brake, the current
through the dynamo is increased, and the lamp again short-circuited.
It has often been pointed out that reversal of the motor on the car
would be a most effective brake. This is certainly true; but, at the
same time, it is a brake that should not be used except in cases of
emergency. For this reason, the dynamo revolving at a high speed, the
momentum of the current is very considerable; hence, owing to the
self-induction of the machine, a sudden reversal will tend to break
down the insulation at any weak point of the machine.
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