e locking-frame of a cabin at Didcot, England,
where an all-electric system has been installed. Wires lead from the
cabin to motors situated at the points and signals, which they operate
through worm gearing. When a lever is moved it closes a circuit and sets
the current flowing through a motor, the direction of the flow (and
consequently of the motor's revolution) depending on whether the lever
has been moved forward or backward. Indicators arranged under the levers
tell the signalman when the desired movements at the points and signals
have been completed. If any motion is not carried through, owing to
failure of the current or obstruction of the working parts, an electric
lock prevents him continuing operations. Thus, suppose he has to open
the main line to an express, he is obliged by the mechanical
locking-frame to set all the points correctly before the signals can be
lowered. He might move all the necessary levers in due order, yet one
set of points might remain open, and, were the signals lowered, an
accident would result. But this cannot happen, as the electric locks
worked by the points in question block the signal levers, and until the
failure has been set right, the signals must remain at "danger."

The point motors are connected direct to the points; but between a
signal motor and its arm there is an "electric slot," consisting of a
powerful electro-magnet which forms a link in the rod work. To lower a
signal it is necessary that the motor shall revolve and a control
current pass round the magnet to give it the requisite attractive force.
If no control current flows, as would happen were any pair of points not
in their proper position, the motor can have no effect on the signal arm
to lower it, owing to the magnet letting go its grip. Furthermore, if
the signal had been already lowered when the control current failed, it
would rise to "danger" automatically, as all signals are weighted to
assume the danger position by gravity. The signal control currents can
be broken by the signalman moving a switch, so that in case of emergency
all signals may be thrown simultaneously to danger.

PNEUMATIC SIGNALLING.

In England and the United States compressed air is also used to do the
hard labour of the signalman for him. Instead of closing a circuit, the
signalman, by moving a lever half-way over, admits air to a pipe running
along the track to an air reservoir placed beside the points or signal
to which the leve