Volume Ii Part 3 (2/2)

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 s.p.a.ce 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 s.p.a.ced 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.

[Ill.u.s.tration: Fig. 339. Face of Magneto Multiple Switchboard]

=Modern Magneto Multiple Board.= Coming now to a consideration of modern magneto multiple switchboards, and bearing in mind that such boards are to be found in modern practice only in comparatively small installations and then only under rather peculiar conditions, as already set forth, we will consider the switchboard of the Monarch Telephone Manufacturing Company as typical of good practice in this respect.

[Ill.u.s.tration: Fig. 340. Monarch Magneto Multiple Switchboard Circuits]

_Line Circuit._ The line and cord circuits of the Monarch system are shown in Fig. 340. It will be seen that each jack has in all five contacts, numbered from _1_ to _5_ respectively, of which _1_ and _4_ are the springs which register with the tip and ring contacts of the plug and through which the talking circuit is continued, while _2_ and _3_ are series contacts for cutting off the line drop when a plug is inserted, and _5_ is the test contact or thimble adapted to register with the sleeve contact on the plug when the plug is fully inserted. The line circuit through the drop may be traced normally from one side of the line through the drop coil, thence through all of the pairs of springs _2_ and _3_ in the jacks of that line, and thence to spring _1_ of the last jack, this spring always being strapped to the spring _2_ in the last jack, and thence to the other side of the line. All the ring springs _1_ are permanently tapped on to one side of the line, and all of the tip springs _4_ are permanently tapped to the other side of the line. This system may, therefore, properly be called a branch-terminal system. It is seen that as soon as a plug is inserted into any of the jacks, the circuit through the drop will be broken by the opening of the springs _2_ and _3_ in that jack. The drop shown immediately above the answering jack is so a.s.sociated mechanically with that jack as to be mechanically self-restored when the answering plug is inserted into the answering jack in response to a call. The arrangement in this respect is the same as that shown in Fig. 259, ill.u.s.trating the Monarch combined drop and jack.

_Cord Circuit._ The cord circuit needs little explanation. The tip and ring strands are the ones which carry the talking current and across these is bridged the double-wound clearing-out drop, a condenser being included in series in the tip strand between the two drop windings in the manner already explained in connection with Fig. 284. The third or sleeve strand of the cord is continuous from plug to plug, and between it and the ground there is permanently connected a r.e.t.a.r.dation coil.

_Test._ The test is dependent on the presence or absence of a path to ground from the test thimbles through some r.e.t.a.r.dation coil a.s.sociated with a cord circuit. Obviously, in the case of an idle line there will be no path to ground from the test thimbles, since normally they are merely connected to each other and are insulated from everything else.

When, however, a plug is inserted into a multiple or answering jack, the test thimbles of that line are connected to ground through the r.e.t.a.r.dation coil a.s.sociated with the third strand of the plug used in making the connection. When the operator applies the tip of the calling plug to a test contact of a multiple jack there will be no path to ground afforded if the line is idle, while if it is busy the potential of the tip of the test plug will cause a current to flow to ground through the impedance coil a.s.sociated with the plug used in making the connection. This will be made clearer by tracing the test circuit. With the listening key thrown this may be traced from the live side of the battery through the r.e.t.a.r.dation coil _6_, which is common to an operator's position, thence through the tip side of the listening key to the tip conductor of the calling cord, and thence to the tip of the calling plug and the thimble of the jack under test. If the line is idle there will be no path to ground from this point and no click will result, but if the line is busy, current will flow from the tip of the test plug to the thimble of the jack tested, thence by the test wire in the multiple to the thimble of the jack at which a connection already exists, and thence to ground through the third strand of the cord used in making that connection and the impedance coil a.s.sociated therewith.

The current which flows in this test circuit changes momentarily the potential of the tip side of the operator's telephone circuit, thus unbalancing her talking circuit and causing a click.

[Ill.u.s.tration: Fig. 341. Magneto Multiple Switchboard]

If this test system were used in a very large board where the multiple would extend through a great many sections, there would be some liability of a false test due to the static capacity of the test contacts and the test wire running through the multiple. For small boards, however, where the multiple is short, this system has proven reliable. A multiple magneto switchboard employing the form of circuits just described is shown in Fig. 341. This switchboard consists of three sections of two positions each. The combined answering jacks and drops may be seen at the lower part of the face of the switchboard and occupying somewhat over one-half of the jack and drop s.p.a.ce. The multiple jacks are above the answering jacks and drops and it may be noted that the same arrangement and number of these jacks is repeated in each section. This switchboard may be extended by adding more sections and increasing the multiple in those already installed to serve 1,600 lines.

_a.s.sembly._ In connection with the a.s.sembly of these magneto multiple switchboards, as installed by the Monarch Company, Fig. 342 shows the details of the cord rack at the back of the board. It shows how the ends of the switchboard cords opposite to the ends that are fastened to the plugs are connected permanently to terminals on the cord rack, at which point the flexible conductors are brought out to terminal clips or binding posts, to which the wires leading from the other portions of the cord circuit are led. In order to relieve the conductors in the cords from strain, the outer braiding of the cord at the rack end is usually extended to form what is called a _strain cord_, and this attached to an eyelet under the cord rack, so that the weight of the cord and the cord weights will be borne by the braiding rather than by the conductors.

This leaves the insulated conductors extending from the ends of the cords free to hang loose without strain and be connected to the terminals as shown. This method of connecting cords, with variations in form and detail, is practically universal in all types of switchboards.

[Ill.u.s.tration: Fig. 342. Cord-Rack Connectors]

A detail of the a.s.sembly of the drops and jacks in such a switchboard is shown in Fig. 343. The single pair of clearing-out drops is mounted in the lower part of the vertical face of the switchboard just above the s.p.a.ce occupied by the plug shelf. Vertical stile strips extend above the clearing-out drop s.p.a.ce for supporting the drops and jacks. A single row of 10 answering jacks and the corresponding line drops are shown in place. Above these there would be placed, in the completely a.s.sembled board, the other answering jacks and line signals that were to occupy this panel, and above these the strips of multiple jacks. The rearwardly projecting pins from the stile strips are for the support of the multiple jack strips, these pins supporting the strips horizontally by suitable multiple clips at the ends of the jack strips; the jack strips being fastened from the rear by means of nuts engaging the screw threads on these pins. This method of supporting drops and jacks is one that is equally adaptable for use in other forms of boards, such as the simple magneto switchboard.

[Ill.u.s.tration: Fig. 343. Drop and Jack Mounting]

[Ill.u.s.tration: Fig. 344. Keyboard Wiring]

In Fig. 344 is shown a detail photograph of the key shelf wiring in one of these Monarch magneto switchboards. In this the under side of the keys is shown, the key shelf being raised on its hinge for that purpose.

The cable, containing all of the insulated wires leading to these keys, enters the s.p.a.ce under the key shelf at the extreme left and from the rear. It then pa.s.ses to the right of this s.p.a.ce where a ”knee” is formed, after which the cable is securely strapped to the under side of the key shelf. By this construction sufficient flexibility is provided for in the cable to permit the raising and lowering of the key shelf, the long reach of the cable between the ”knee” and the point of entry at the left serving as a torsion member, so that the raising of the shelf will give the cable a slight twist rather than bend it at a sharp angle.

CHAPTER XXVI

THE COMMON-BATTERY MULTIPLE SWITCHBOARD

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