ARTHUR H. BELL

Believing that amateurs who have followed this series of articles, and have made experiments in connection with them, would like to become familiar with the rudiments of practical telephony and continue, step by step, to the higher branches of the art, the writer will describe some of the simple apparatus and connections common to the many systems.

It is the custom, in certain types of exchanges, to signal the central office by turning the crank of the magneto generator of your instrument. The current generated passes over the line wire to the central switchboard, where it energizes an electro magnet device called a "drop ". There are several styles and shapes of drops, but the fundamental principal is that of the electro magnet, wound with very fine wire to a resistance of 500 or more ohms, as the requirements may be.

Fig. 1 illustrates a drop. The shutter a is locked in place by the trigger or tongue b, which is part of the armature c. When the magnet becomes energized the armature, which nominally is a small fraction of an inch from the core, and consequently the tongue attached to it is moved upward an eighth of an inch or so. The shutter is inclined somewhat outward and drops when released by the action of the armature.

When the drop falls the operator places a "plug," connected with the operator's instrument, in the "jack " and talks with the party calling.

Fig. 2 illustrates a plug and a jack. The plug is, for convenience, built of round brass rod, so that it requires no particular position when entering the jack. Earlier styles of plugs were constructed of two flat strips insulated with hard wood or rubber, but these are now seldom used except in telegraph circuits.

It will be seen that when the plug, with its conductiug pair of flexible wires, is inserted in the jack, which likewise has connection to a pair of wires, that whatever may be electrically connected on the plug side is given connection with whatever may be affixed to the jack wires, and the means of connecting or disconnecting is controlled by the hand that manipulates the plug. And if there were two distinct lines, each ending in a jack, and a cord bearing a plug at each end was inserted the two jacked lines would be given metallic connection with each other through the plugs and flexible cords. Flexible cords are made of tinsel, like electric light cord, for example, and electrically are equal in conductivity to solid wire which, of course, could not be util ized for the purpose.

If we were to devise an experimental central office for the purpose of studying the early principles of switchboards, we would require a special plug and cord attached to the central operating instrument, with which calls would be answered. The plug would then be removed and the subscriber conneted with the line called for by the double plug and cord device. Such an arrangement would not suffice in actual telephony at the present time, yet such was the method used up to the day the "key " was introduced, a score or more years ago. A key is a switching device, and its purposes are many. Fig. 3 illustrates a key adaptable to the foregoing circuits. By pressing down the button the rubber plunger is forced against the two inside springs, to which are affixed wires leading from the telephone used by the operator. To the outside springs are connected the plugs and cord device mentioned in the last paragraph. Further pressure on the plunger forces the inside springs against the outer, and places the operator's telephone in direct communication with the station jacks in which she may insert the plug or plugs. When pressure is removed from the key the operators' set is removed from the cord circuit.