Cyclopedia of Telephony and Telegraphy, Vol. 2 / A General Reference Work on Telephony, etc. etc.

by American School of Correspondence

The subscriber's hook switch is mechanically interlocked with the dial so as to prevent the dial being moved from its normal position until the hook is in its raised position. This interlocking function involves also the pivoted dog 4. Normally the lower end of this dog lies in the path of the pin 5 carried on the lever 2, and thus the shaft, dial, and segment are prevented from any considerable movement when the receiver is on the hook. However, when the receiver is removed from its hook, the upwardly projecting arm from the hook engages a projection on the dog 4 and moves the dog out of the path of the pin 5. Thus the dial is free to be rotated by the subscriber. The pin 6 is mounted in a stationary position and serves to limit the backward movement of the dial by the lever 2 striking against it.

Ground Springs:—Five groups of contact springs must be considered, some of which are controlled wholly by the position of the switch hook, others jointly by the position of the switch hook and the dial, others by the movement of the dial itself, and still others by the pressure of the subscriber's finger on a button. The first of these groups consists of the springs 7 and 8, the function of which is to control the continuity of the ground connection at the subscriber's station. The arrangement of these two springs is such that the ground connection will be broken until the subscriber's receiver is removed from its hook. As soon as the receiver is raised, these springs come together in an obvious manner, the dog 4 being lifted out of the way by the action of the hook. The ledge on the lower portion of the spring 7 serves as a rest for the insulated arm of the dog 4 to prevent this dog, which is spring actuated, from returning and locking the dial until after the receiver has been hung up.

Bell and Transmitter Springs:—The second group is that embracing the springs 9, 10, 11, and 12. The springs 10 and 11 are controlled by the lower projection from the switch hook, the spring 11 engaging the spring 12 only when the hook is down. The spring 10 engages the spring 9 only when the hook lever is up and not then unless the dial is in its normal position. While the hook is raised, therefore, the springs 9 and 10 break contact whenever the dial is moved and make contact again when it returns to its normal position. The springs 11 and 12 control the circuit through the subscriber's bell while the springs 9 and 10 control the continuity of the circuit from one side of the line to the other so as to isolate the limbs from each other while the signal transmitter is sending its impulses to the central office.

Impulse Springs:—The third group embraces springs 13, 14, and 15 and these are the ones by which the central-office switches are controlled in building up a connection.

Something of the prevailing nomenclature which has grown up about the automatic system may be introduced at this point. The movements of the selecting switches at the central office are referred to as vertical and rotary for obvious reasons. On account of this the magnet which causes the vertical movement is referred to as the vertical magnet and that which accomplishes the rotary movement as the rotary magnet. It happens that in all cases the selecting impulses sent by the subscriber's station, corresponding respectively to the number of digits in the called subscriber's number, are sent over one side of the line and in nearly all cases these selecting impulses actuate the vertical movements of the selecting switches. For this reason the particular limb of the line over which the selecting impulses are sent is called the vertical limb. The other limb of the line is the one over which the single impulse is sent after each group of selecting impulses, and it is this impulse in every case which causes the selector switch to start rotating in its hunt for an idle trunk. This side of the line is, therefore, called rotary. For the same reasons the impulses over the vertical side of the line are called vertical impulses and those over the rotary side, rotary impulses. The naming of the limbs of the line and of the current impulses vertical and rotary may appear odd but it is, to say the least, convenient and expressive.

Coming back to the functions of the third group of springs, 13, 14, and 15, 15 may be called the vertical spring since it sends vertical impulses; 13, the rotary spring since it sends rotary impulses; and 14, the ground spring since, when the hook is up, it is connected with the ground.

On the segment 3 there are ten projections or cams 16 which, when the dial is moved, engage a projection of the spring 15. When the dial is being pulled by the subscriber's finger, these cams engage the spring 15 in such a way as to move it away from the ground spring and no electrical contact is made. On the return of the dial, however, these cams engage the projection on the spring 15 in the opposite way and the passing of each cam forces this vertical spring into engagement with the ground spring. It will readily be seen, therefore, by a consideration of the spacing of these cams on the segment and the finger holes in the dial that the number of cams which pass the vertical spring 15 will correspond to the number on the hole used by the subscriber in moving the dial.

Near the upper right-hand corner of the segment 3, as shown in Fig. 384, there is another projection or cam 17, the function of which is to engage the rotary spring 13 and press it into contact with the ground spring. Thus, the first thing that happens in the movement of the dial is for the projection 17 to ride over the hump on the rotary spring and press the contact once into engagement with the ground spring; and likewise, the last thing that happens on the return movement of the dial is for the rotary spring to be connected once to the ground spring after the last vertical impulse has been sent.

If both the rotary and vertical sides of the line are connected with the live side of the central-office battery, it follows that every contact between the vertical and the ground spring or between the rotary and the ground spring will allow an impulse of current to flow over the vertical or the rotary side of the line.

We may summarize the action of these impulse springs by saying that whenever the dial is moved from its normal position, there is, at the beginning of this movement, a single rotary impulse over the rotary side of the line; and that while the dial returns, there is a series of vertical impulses over the vertical side of the line; and just before the dial reaches its normal position, after the sending of the last vertical impulse, there is another impulse over the rotary side of the line.