Fig. 338. Branch-Terminal Magneto Multiple Switchboard
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Each jack has five contacts, and the answering and multiple jacks are alike, both in respect to their construction and their connection with the line. The drops are the electrically self-restoring type shown in Fig. 263. The line circuits extended permanently from the subscriber's station to the line winding of the drop and the two limbs of the line branched off to the tip and sleeve contacts 1 and 2 respectively of each jack. Another pair of wires extended through the multiple parallel to the line wires and these branched off respectively to the contact springs 3 and 4 of each of the jacks. This pair of wires formed portions of the drop-restoring circuit, including the restoring coil 6 and the battery 7, as indicated. The test thimble 5 of each of the jacks is connected permanently with the spring 3 of the corresponding jack and, therefore, with the wire which connects through the restoring coil 6 of the corresponding drop to ground through the battery 7.

The plugs were each provided with three contacts. Two of these were the usual tip and sleeve contacts connected with the two strands of the cord circuit. The third contact 8 was not connected with any portion of the cord circuit, being merely an insulated contact on the plug adapted, when the plug was fully inserted, to connect together the springs 3 and 4. The cord circuit itself is readily understood from the drawing, having two features, however, which merit attention. One is the establishing of a grounded battery connection to the center portion of the winding of the receiver for the purposes of the busy test, and the other is the provision of a restoring coil and restoring circuit for the clearing-out drop, this circuit being closed by an additional contact on the listening key so as to restore the clearing-out drop whenever the listening key was operated.

Operation. An understanding of the operation of this system is easy. The turning of the subscriber's generator, when the line was in its normal condition, caused the display of the line signal. The insertion of the answering plug, in response to this call, did three things: (1) It extended the line circuit to the tip and sleeve strand of the cord circuit. (2) It energized the restoring coil 6 of the drop by establishing the circuit from the contact spring 3 through the plug contact 8 to the other contact spring 4, thus completing the circuit between the two normally open auxiliary wires. (3) The connecting of the springs 3 and 4 established a connection from ground to the test thimbles of all the jacks on a line, the spring 4 being always grounded and the spring 3 being always connected to the test thimble 5.

It is to be noted that on idle lines the test rings are always at the same potential as the ungrounded pole of the battery 7, being connected thereto through the winding 6 of the restoring coil. On all busy lines, however, the test rings are dead grounded through the contact 8 of the plug that is connected with the line.

The tip of the testing plug at the time of making a test will also be at the same potential as that of the ungrounded pole of the battery 7, since this pole of the battery 7 is always connected to the center portion of the operator's receiver winding, and when the listening key is thrown the tip of the calling plug is connected therewith and is at the same potential. When, therefore, the operator touches the tip of the calling plug to the test thimble of an idle line, she will get no click, since the tip of the plug and the test thimble will be at the same potential. If, however, the line has already been switched at another section of the board, there will be a difference of potential, because the test thimble will be grounded, and the circuit, through which the current which causes the click flows, may be traced from the ungrounded pole of the battery 7 to the center portion of the operator's receiver, thence through one-half of the winding to the tip of the calling plug, thence to the test thimble of the jack under test, thence to the spring 3 of the jack on another section at which the connection exists, through the contact 8 on the plug of that jack to the spring 4, and thence to ground and back to the other terminal of the battery 7.

Magnet Windings. Coils of the line and clearing-out drops by which these drops are thrown, are wound to such high resistance and impedance as to make it proper to leave them permanently bridged across the talking circuit. The necessity for cutting them out is, therefore, done away with, with a consequent avoidance, in the case of the line drops, of the provision of series contacts in the jacks.

Arrangement of Apparatus. In boards of this type the line and clearing-out drops were mounted in the extreme upper portion of the switchboard face so as to be within the range of vision of the operator, but yet out of her reach. Therefore, the whole face of the board that was within the limit of the operator's reach was available for the answering and multiple jacks. A front view of a little over one of the sections of the switchboard, involving three complete operator's positions, is shown in Fig. 339, which is a portion of the switchboard installed by the Western Electric Company in one of the large exchanges in Paris, France. (This has recently been replaced by a common-battery multiple board.) In this the line drops may be seen at the extreme top of the face of the switchboard, and immediately beneath these the clearing-out drops. Beneath these are the multiple jacks arranged in banks of one hundred, each hundred consisting of five strips of twenty. At the extreme lower portion of the jack space are shown the answering jacks and beneath these on the horizontal shelf, the plugs and keys. These jacks were mounted on 1/2-inch centers, both vertically and horizontally and each section had in multiple 90 banks of 100 each, making 9,000 in all. Subsequent practice has shown that this involves too large a reach for the operators and that, therefore, 9,000 is too large a number of jacks to place on one section if the jacks are not spaced closer than on 1/2-inch centers. With the jack involving as many parts as that required by this branch terminal system, it was hardly feasible to make them smaller than this without sacrificing their durability, and one of the important features of the common-battery multiple system which has supplanted this branch-terminal magneto system is that the jacks are of such a simple nature as to lend themselves to mounting on 3/8-inch centers, and in some cases on 3/10-inch centers.