Assuming both keys to be at rest, as in Fig. 33, the two buzzers are 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 contact, 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.