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. The action is
analogous to the spark produced by a Ruhmkorff coil. This was
illustrated at Portrush; when the car was running perhaps fifteen
miles an hour, the current was suddenly reversed. The car came to a
standstill in little more than its own length, but at the expense of
breaking down the insulation of one of the wires of the magnet coils.
The way out of the difficulty is evidently at the moment of reversal
to insert a high resistance to diminish the momentum of the current.

In determining the proper dimensions of a conductor for railway
purposes, Sir William Thomson's law should properly apply. But on a
line where the gradients and traffic are very irregular, it is
difficult to estimate the average current, and the desirability of
having the rail mechanically strong, and of such low resistance that
the potential shall not vary very materially throughout its length,
becomes more important than the economic considerations involved in
Sir William Thomson's law. At Portrush the resistance of a mile,