connected directly with the brass strip r of the spring jack, which terminates in a curved end. The other side of the line circuit is joined to the brass spring b, the end of which is also curved as shown. The spring b has a contact point which, when in its normal position, presses against the spring g, and r is similarly connected to the spring f. The line drop A is joined in circuit by wires connecting its magnet winding to the springs ƒ and g.

A party desiring a connection with some other party signals the switchboard operator at the cen

FIG. 115. Each Line Circuit Terminates in the Exchange Switchboard as here Shown

tral exchange by operating his magneto generator; this sends a current through the magnet winding of the line drop A connected to his circuit at the switchboard, and, by attracting the armature u, releases a catch which permits the shutter o to drop. The swinging down of the shutter attracts the opcrator's attention and reveals to her the number of the calling party previously hidden by the shutter.

The Operator's Circuit on the switchboard is shown in Fig. 116. The two plugs m n and r v have their tips m and r electrically connected to the respective springs s and i of the ringing keys R and

R', and their sleev s n and v, which are insulated from their tips, are connected respectively to the springs e and c. The inner contacts of the springs of one ringing key are connected to the inner contacts of the springs of the other ringing key, and between the wires leading from the outer contacts of both keys is connected the magneto generator At the points d and h, in the circuit joining

g.

p

HL

R

b

d

h

g

FIG. 116.—Wiring of the Operator's Circuit on an Exchange Switchboard

the ringing keys, wires run to the outer contacts of the key L, called the operator's listening key, and between these wires is the clearing-out drop b. The springs of the key L are connected with the operator's receiver o and secondary winding j of an induction coil, the primary winding of which is in circuit with the battery y and operator's transmitter p. In the switchboard there is but I primary circuit ly p, 1 secondary circuit o j, and I magneto generator g, but the rest of the appara

tus and wiring in Fig. 116 is repeated for each operator's circuit; in the switchboard, Fig. 114, there are, as previously noted, 10 operator's circuits, enabling 20 different parties to be connected in pairs simultaneously.

A party having signaled the operator as already described, the first duty of the operator is to find from him the number of the party desired. This the operator does by inserting the plug m n, called the answering plug, into the calling party's jack S, Fig. 115. As all the jacks and line drops are numbered, those connected to the party's telephone bearing the same number as his line circuit. and telephone set, it is an easy matter for the operator to select the proper jack in which to insert her plug. When inserted and pushed in, the tip m of the plug makes contact with the curved part of the spring r, and the sleeve ʼn makes contact with the curved part of the spring b. As the springs r and bare forced outward by the plug they separate from the springs ƒ and g and thus cut out of circuit the line drop A. The operator next closes her listening key L, Fig. 116, so that its springs touch the outer contacts; this brings her telephone in circuit with the calling party's line and enables her to learn from him the number of the party with whom he desires to speak.

Having been informed of the desired number, the operator introduces her plug rv, called the answering plug, into the jack containing the number asked for, and closes the ringing key R'.

When this key is closed and the magneto generator g put into operation, current from the generator passes over the line and rings the bell in the desired party's telephone set, attracting his attention. No current will flow elsewhere because the inner contacts of the ringing key R' are opened when this key is closed. The party signaled, upon answering, finds himself in connection with the party who desires to talk with him. As soon as the conversation has been started, the operator opens her listening key L so as to leave her telephone circuit free for answering other calls. When the parties connected have finished their conversation, they hang up their receivers and give the handles of their magneto generators a few turns; this generates the current required to actuate the clearingout drop b, which, when its shutter falls, acts as a signal to the operator that the parties have finished talking. When the operator receives this ring off" signal, she withdraws the plugs m n and rv from the jacks in which they were placed.

In the diagram, Fig. 116, it will be noted two ringing keys R and R' are indicated, but that only R' was used in the foregoing procedure. The ringing key R is needed only when the plug m n is used as the answering plug and rv as the calling plug, instead of vice versa. As it is not convenient always to use the same plug for the same purpose, the ringing key R is often dispensed with and the listening key L and the ringing key R' combined into one as in the switchboard, Fig. 114.

A Combination Ringing and Listening Key as made by the International Telephone Manufacturing Company is shown in Fig. 117. The handle h is all that projects above the switchboard shelf s, the springs, etc., being mounted below. The one set of springs form the ringing contacts, and the other set the listening contacts. Pressing the handle forward or backward closes the one set

FIG. 117.-Combination Ringing and Listening Key

of springs or the other by reason of the insulated knob a or c forcing them together on cach side.

Spring Jacks and Plugs as made by the Central Electric Company are shown respectively in Figs. 118 and 119. It is readily seen that, when the tip m of the plug is inserted through the hole in the part n of the jack and comes in contact with the spring r, how the sleeve s of the plug comes in contact with the spring b; how, when the plug is