Volume Ii Part 5 (2/2)

_Cord Circuit._ The cord circuit employed is of the two-conductor type, the plugs being so constructed as to connect the ring and thimble contacts of the jack when inserted. This cord circuit is somewhat similar to that employed by the Kellogg Switchboard and Supply Company, shown in Fig. 352, except that only one battery is employed, and that certain functions of this circuit are performed mechanically by the inter-action of the armatures of the relays.

_Supervisory Signals._ When the answering plug is inserted in a jack, in response to a call, the current pa.s.sing to the subscriber's station and also through the cut-off relay must flow through the relay _1_, thus energizing it. As the calling subscriber's receiver is at this time removed from the hook switch, the path for current will be completed through the tip of the jack, thence through the tip of the plug, through relay _2_ to ground, causing relay _2_ to be operated and to break the circuit of the answering supervisory lamp. The two relays _1_ and _2_ are so a.s.sociated mechanically that the armature of _1_ controls the armature of _2_ in such a manner as to normally hold the circuit of the answering supervisory lamp open. But, however, when the plug is inserted in a jack, relay _1_ is operated and allows the operation of relay _2_ to be controlled by the hook switch at the subscriber's station. The supervisory relay _3_ a.s.sociated with the calling cord is operated when the calling plug is placed in a jack, and this relay normally holds the armature of relay _4_ in an operated position in a similar manner as the armature of relay _1_ controlled that of relay _2_. Supervisory relay _4_ is under the control of the hook switch at the called subscriber's station.

_Test._ In this circuit, as in several previously described, when a plug is inserted in a jack of a line, the thimble contacts of the jacks a.s.sociated with that line are raised to a higher potential than that which they normally have. The operator in testing a busy line, of course having previously moved the listening key to the listening position, closes a path from the test thimble of the jack, through the tip of the calling plug, through the contacts of the relay _4_, the inside springs of the listening key, thence through a winding of the induction coil a.s.sociated with her set to ground. The circuit thus established allows current to flow from the test thimble of the jack through the winding of her induction coil to ground, causing a click in her telephone receiver.

The arrangement of the ringing circuit does not differ materially from that already described for other systems and, therefore, needs no further explanation.

[Ill.u.s.tration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]

=Multiple Switchboard Apparatus.= Coming now to a discussion of the details of apparatus employed in multiple switchboards, it may be stated that much of the apparatus used in the simpler types is capable of doing duty in multiple switchboards, although, of course, modification in detail is often necessary to make the apparatus fit the particular demands of the system in which it is to be used.

_Jacks._ Probably the most important piece of apparatus in the multiple switchboard is the jack, its importance being increased by the fact that such very large numbers of them are sometimes necessary. Switchboards having hundreds of thousands of jacks are not uncommon. The multiple jacks are nearly always mounted in strips of twenty and the answering jacks usually in strips of ten, the length of the jack strip being the same in each case in the same board and, therefore, giving twice as wide a s.p.a.cing in the answering as in the multiple jacks. The distance between centers in the multiple jacks varies from a quarter of an inch--which is perhaps the extreme minimum--to half an inch, beyond which larger limit there seems to be no need of going in any case. It is customary that the jack strip shall be made of the same total thickness as the distance between the centers of two of its jacks, and from this it follows that the strips when piled one upon the other give the same vertical distance between jack centers as the horizontal distance.

In Fig. 358 is shown a strip of multiple and a strip of answering jacks of Western Electric make, this being the type employed in the No. 1 standard switchboards for large exchanges. In Fig. 359 are shown the multiple and answering jacks employed in the No. 10 Western Electric switchboard. The multiple jacks in the No. 1 switchboard are mounted on 3/8-inch centers, the jacks having three branch terminal contacts. The multiple jacks of the No. 10 switchboard indicated in Fig. 359 are mounted on 1/2-inch centers, each jack having five contacts as indicated by the requirement of the circuits in Fig. 349.

In Fig. 360 are shown the answering and multiple jacks of the Kellogg Switchboard and Supply Company's two-wire system. The extreme simplicity of these is particularly well shown in the cut of the answering jack, and these figures also show clearly the customary method of numbering jacks. In very large multiple boards it has been the practice of the Kellogg Company to s.p.a.ce the multiple jacks on 3/10-inch centers, and in their smaller multiple work, they employ the 1/2-inch s.p.a.cing. With the 3/10-inch s.p.a.cing that company has been able to build boards having a capacity of 18,000 lines, that many jacks being placed within the reach of each operator.

In all modern multiple switchboards the test thimble or sleeve contacts are drawn up from sheet bra.s.s or German silver into tubular form and inserted in properly s.p.a.ced borings in strips of hard rubber forming the faces of the jacks. These strips sometimes are reinforced by bra.s.s strips on their under sides. The springs forming the other terminals of the jack are mounted in milled slots in another strip of hard rubber mounted in the rear of and parallel to the front strip and rigidly attached thereto by a suitable metal framework. In this way desired rigidity and high insulation between the various parts is secured.

[Ill.u.s.tration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]

_Lamp Jacks._ The lamp jacks employed in multiple work need no further description in view of what has been said in connection with lamp jacks for simple common-battery boards. The lamp jack s.p.a.cing is always the same as the answering jack s.p.a.cing, so that the lamps will come in the same vertical alignment as their corresponding answering jacks when the lamp strips and answering jack strips are mounted in alternate layers.

[Ill.u.s.tration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]

[Ill.u.s.tration: Fig. 360. Answering and Multiple Jacks for Kellogg Two-Wire Board]

_Relays._ Next in order of importance in the matter of individual parts for multiple switchboards is the relay. The necessity for reliability of action in these is apparent, and this means that they must not only be well constructed, but that they must be protected from dust and moisture and must have contact points of such a nature as not to corrode even in the presence of considerable sparking and of the most adverse atmospheric conditions. Economy of s.p.a.ce is also a factor and has led to the almost universal adoption of the single-magnet type of relay for line and cut-off as well as supervisory purposes.

[Ill.u.s.tration: Fig. 361. Type of Line Relay]

[Ill.u.s.tration: Fig. 362. Type of Cut-Off Relay]

The Western Electric Company employs different types of relays for line, cut-off, and supervisory purposes. This is contrary to the practice of most of the other companies who make the same general type of relay serve for all of these purposes. A good idea of the type of Western Electric line relay, as employed in its No. 1 board, may be had from Fig. 361. As is seen this is of the tilting armature type, the armature rocking back and forth on a knife-edge contact at its base, the part on which it rests being of iron and of such form as to practically complete, with the armature and core, the magnetic circuit. The cut-off relay, Fig. 362, is of an entirely different type. The armature in this is loosely suspended by means of a flexible spring underneath two L-shaped polar extensions, one extending up from the rear end of the core and the other from the front end. When energized this armature is pulled away from the core by these L-shaped pieces and imparts its motion through a hard-rubber pin to the upper pair of springs so as to effect the necessary changes in the circuit.

[Ill.u.s.tration: Fig. 363. Western Electric Combined Line and Cut-off Relay]

[Ill.u.s.tration: Fig. 364. Western Electric Supervisory Relay]

[Ill.u.s.tration: Fig. 365. Line Relay No. 10 Board]

Much economy in s.p.a.ce and in wiring is secured in the type of switchboards employing cut-off as well as line relays by mounting the two relays together and in making of them, in fact, a unitary piece of apparatus. Since the line relay is always a.s.sociated with the cut-off relay of the same line and with no other, it is obvious that this unitary arrangement effects a great saving in wiring and also secures a great advantage in the matter of convenience of inspection. Such a combined cut-off and line relay, employed in the Western Electric No. 1 relay board, is shown in Fig. 363. These are mounted in banks of ten pairs, a common dust cap of sheet iron covering the entire group.

The Western Electric supervisory relay, Fig. 364, is of the tilting armature type and is copper clad. The dust cap in this case fits on with a bayonet joint as clearly indicated. In Fig. 365 is shown the line relay employed in the Western Electric No. 10 board.

[Ill.u.s.tration: Fig. 366. Kellogg Line and Cut-off Relays]

[Ill.u.s.tration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]

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