Fig. 388. Line-Switch Unit
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Fig. 389. Circuits of Line-Switch Unit
[View full size illustration.]

The vertical and rotary sides of the subscriber's line are shown entering at the lower left-hand corner of this figure, and they pass to the springs of the contact bank. Immediately adjacent to these springs are the trunk contacts from which the vertical and the rotary limbs of the first selector trunk proceed. The plunger is indicated at 1, it being in the form of a wheel of insulating material. It is carried on the rod 2 pivoted on a lever 3, which, in turn, is pivoted at 4 in a stationary portion of the framework. A spring 5, secured to the underside of the lever 3 and projecting to the left beyond the pivot 4 of this lever, serves always to press the right-hand portion of the lever 3 forward in such direction as to tend to thrust it into the contact bank. The plunger is normally held out of the contact bank by means of the latch 6 carried on the armature 7 of the trip magnet. When the trip magnet is energized it pulls the armature 7 to the left and thus releases the plunger and allows it to enter the contact bank.

The master bar is shown at 8, and a feather on this bar engages a notch in the segment attached to the rear end of the plunger rod 2. This master bar is common to all of the plunger rods and by its oscillatory movement, under the influence of the master switch, it always keeps all of the idle plunger bars pointed toward the contacts of an idle trunk. As soon, however, as the trip magnet is operated to cause the insertion of a plunger into the contact bank, the feather on the master bar is disengaged by the notch in the segment of the plunger rod, and the plunger rod is, therefore, no longer subject to the oscillating movement of the master bar.

When the release magnet is energized, it attracts its armature 9 and this lifts the armature 7 of the trip magnet so that the latch 6 rides on top of the left-hand end of the lever 3. Then, when the release magnet is de-energized, the spring 5, which was put under tension by the latch, moves the entire structure of levers back to its normal position, withdrawing the plunger from the bank of contacts. The notch on the edge of the segment of the plunger rod, when thus released, will probably not strike the feather on the master bar, and the plunger rod will thus not come under the control of the master bar until the master bar has moved, in its oscillation, so that the feather registers with the notch, after which this bar will move with all the others.

If, while the plunger is waiting to be picked up by the master bar, the same subscriber should call again, his line will be connected with the same trunk as before. There is no danger in this, however, that the trunk will be found busy, because the master bar will not have occupied a position which would make it possible for any of the lines to appropriate this trunk during the intervening time.

Master Switch. Associated with each master bar there is a master switch which determines the position in which the master bar shall stop in order that the idle plungers may be pointed always to the contacts of an idle trunk. The arm 10 of this switch is attached to the master bar and oscillates with it and serves to connect the segment 11 successively with the contacts 12, which are connected respectively to the third, or release wire of each first selector trunk. In the figure the arm 10 is shown resting on the sixth contact of the switch and this sixth contact is connected to a spring 13 in the line-switch contact bank that has not yet been referred to. As soon as the plunger is inserted into the contact bank, the spring 14 will be pressed into engagement with the spring 13, and this spring 14 is connected with the live side of the battery through the release magnet winding.

The contact strip 11 on the master switch is thus connected through the release magnet to the battery and from this current flows through the left-hand winding of the master-switch relay. This energizes this relay and causes the closure of the circuit of the locking magnet which magnet unlocks the master bar to permit its further rotation. The unlocking of the master bar brings the spring 15 into engagement with 16 and thus energizes the master magnet, the armature of which vibrates back and forth after the manner of an electric-bell armature, and steps the wheel 17 around. The wheel 17 is mechanically connected to the master bar so that each complete revolution of the wheel will cause one complete oscillation of the master bar. The master bar will thus be moved so as to cause all the idle plungers to sweep through an arc and this movement will stop as soon as the master-switch arm 10 connects the arc 11 with one of the contacts 12 that is not connected to the live side of the battery through the springs 13 and 14 of some other line switch. It is by this means that the plungers of the line switches are always kept pointing at the contacts of an idle trunk. The way in which this feature has been worked out must demand admiration and accounts for the marvelous quickness of this line switch. The fact that the plungers are pointed in the right direction before the time comes for their use, leaves only the simple thrusting motion of the plunger to accomplish the desired connection immediately upon the initiation of a call by the subscriber.

Locking Segment. It will be understood that the locking segment 18 and the master-switch contact finger 10 are both rigidly connected with the master bar 8 and move with it, the locking segment 18 serving always to determine accurately the angular position at which the master bar and the master-switch arm are brought to rest.