evidently in circuit with the line wires, though no current is passing. If
the stem of K is depressed to make contact with M, the electric circuit of
which the battery, B, forms part is completed, and the buzzer at the other
end of the lines comes into action. Since the depression of K raises O off
N, the "home" buzzer's connection with the line wires is broken, to prevent
the current being short-circuited. The fact that this buzzer is
periodically in circuit, even when the key is being worked, makes it
possible for the operator at the other end to attract attention by
depressing his key, if he cannot read the signals sent.

[Illustration: Fig.33--Telegraphic apparatus; sending key, buzzer and
battery]

Making the Keys.

Transmitting keys can be bought cheaply, but not so cheaply as they can be
made. The only expense entailed in home manufacture is that of the screw
terminals for connecting the keys with the lines and buzzers. These cost
only a penny each, and, if strict economy is the order of the day, can be
dispensed with should the apparatus not have to be disconnected frequently.

The size of the key is immaterial. The keys made by me have levers 1 inch
wide and 5-1/2 inches long, oak being chosen as material, on account of its
toughness. K is in each case a small wooden knob on a piece of 3/16-inch
brass rod; O a 1-1/2-inch brass screw; A a piece of sheet brass 3-1/2
inches long, marked off carefully, drilled 1/8 inch from the centre of each
end for the pivot screws, and in four places for the holding-down screws,
and bent up at the ends to form two standards. If you do not possess any
brass strip, the lever may be supported on wooden uprights glued and
screwed to the base.

[Illustration: Fig. 34--Telegraphic apparatus mounted on baseboard]

Contact M is a small piece of brass attached to the base by a screw at one
end and by T1 at the other. K was drilled near the end to take the short
coil of insulated wire joining it to T2, and O was similarly connected with
T4.

The spring, S, should be fairly strong. A steel spiral with a loop at each
end is most easily fitted. Drill holes in the lever and base large enough
for the spring to pass through freely, make a small cross hole through the
lever hole for a pin, and cut a slot across the base hole for a pin to hold
the bottom of the spring. Adjust the lever by means of screw O so that
there is a space of about 1/4-inch between K and M when O and N are in
cont