act, and after the spring has been put in position give the screw a
turn or two to bring K down to within 1/16 inch of M. This will put the
required tension on the spring.

The Buzzers.--For these I selected a couple of small electric bells,
costing 2s. 6d. each. Their normal rate of vibration being much too slow
for telegraphic purposes, I cut off the hammers to reduce the inertia, and
so adjusted the contact screw that the armature had to move less than one
hundredth of an inch to break the circuit. This gave so high a rate of
vibration that the key could not make and break the circuit quickly enough
to prevent the buzzer sounding.

A Morse Tapper or Sounder.

In postal telegraph offices a "sounder," and not a "buzzer," is generally
used to communicate the signals. Instead of a continuous noise, lasting as
long as the key at the transmitting station is held down, the operator at
the receiving station hears only a series of taps made by an instrument
called a "sounder." The principle of this simple device is illustrated by
the working diagrams in Fig. 35. M is a horseshoe magnet fixed to a base,
A. Close to it is an armature, AR, of soft iron, attached to a lever, L,
which works on a pivot and is held up against a regulating screw, P1, by
the pull of the spring SP. When current passes through the magnet the
armature is attracted, and the point of the screw S2 strikes against P2;
while the breaking of the circuit causes L to fly back against S1. The time
intervening between the "down" and "up" clicks tells the operator whether a
long or a short--dash or a dot--is being signalled.

[Illustration: FIG. 35.-Elevation and plan of telegraphic sounder.]

Materials.--A horseshoe magnet and armature taken from an electric bell
provide the most essential parts of our home-made instrument in a cheap
form. If these are available, expense will be limited to a few pence. Oak
or walnut are the best woods to use for the lever, being more resonant than
the softer woods, and for the standard B and stop V. Any common wood is
good enough for the base A.

The lever L is 6 inches long, 1/2 inch deep, and 3/8-inch wide, and is
pivoted at a point 4-1/4 inches from the stop end. The hole should be bored
through it as squarely as possible, so that it may lie centrally without B
being out of the square. A piece of metal is screwed to its top face under
the adjusting screw S1.

The spring is attached to L and A in the manner already desc