Volume I Part 30 (1/2)

A feature that is made much of in recently designed drops and jacks for magneto service is that which provides for the ready removal of the drop coil, from the rest of the structure, for repair. The drop and jack of the Western Electric Company, just described, embodies this feature, a single screw being so arranged that its removal will permit the withdrawal of the coil without disturbing any of the other parts or connections. The coil windings terminate in two projections on the front head of the spool, and these register with spring clips on the inside of the sh.e.l.l so that the proper connections for the coil are automatically made by the mere insertion of the coil into the sh.e.l.l.

[Ill.u.s.tration: Fig. 254. Western Electric Drop and Jack]

[Ill.u.s.tration: Fig. 255. Western Electric Drop and Jack]

Dean Type:--The combined drop and jack of the Dean Electric Company is ill.u.s.trated in Figs. 256 and 257. The two perspective views show the general features of the drop and jack and the method by which the magnet coil may be withdrawn from the sh.e.l.l. As will be seen the magnet is wound on a hollow core which slides over the iron core, the latter remaining permanently fixed in the sh.e.l.l, even though the coil be withdrawn.

Fig. 258 shows the structural details of the jack employed in this combination and it will be seen that the restoring spring for the drop is not the tip spring itself, but another spring located above and insulated from it and mechanically connected therewith.

[Ill.u.s.tration: Fig. 256. Dean Drop and Jack]

[Ill.u.s.tration: Fig. 257. Dean Drop and Jack]

[Ill.u.s.tration: Fig. 258. Details of Dean Jack]

Monarch Type:--Still another combined drop and jack is that of the Monarch Telephone Manufacturing Company of Chicago, shown in sectional view in Fig. 259. This differs from the usual type in that the armature is mounted on the front end of the electromagnet, its latch arm retaining the shutter in its normal position when raised, and releasing it when depressed by the attraction of the armature. As is shown, there is within the core of the magnet an adjustable spiral spring which presses forward against the armature and which spring is compressed by the attraction of the armature of the magnet. The night-alarm contact is clearly shown immediately below the strip which supports the drop, this consisting of a spring adapted to be engaged by a lug on the shutter and pressed upwardly against a stationary contact when the shutter falls. The method of restoration of the shutter in this case is by means of an auxiliary spring bent up so as to engage the shutter and restore it when the spring is raised by the insertion of a plug into the jack.

[Ill.u.s.tration: Fig. 259. Monarch Drop and Jack]

_Code Signaling._ On bridging party lines, where the subscribers sometimes call other subscribers on the same line and sometimes call the switchboard so as to obtain a connection with another line, it is not always easy for the operator at the switchboard to distinguish whether the call is for her or for some other party on the line. On such lines, of course, code ringing is used and in most cases the operator's only way of distinguis.h.i.+ng between calls for her and those for some sub-station parties on the line is by listening to the rattling noise which the drop armature makes. In the case of the Monarch drop the adjustable spring tension on the armature is intended to provide for such an adjustment as will permit the armature to give a satisfactory buzz in response to the alternating ringing currents, whether the line be long or short.

[Ill.u.s.tration: Fig. 260. Code Signal Attachment]

The Monarch Company provides in another way for code signaling at the switchboard. In some cases there is a special attachment, shown in Fig. 260, by means of which the code signals are repeated on the night-alarm bell. This is in the nature of a special attachment placed on the drop, which consists of a light, flat spring attached to the armature and forming one side of a local circuit. The other side of the circuit terminates in a fixture which is mounted on the drop frame and is provided with a screw, having a platinum point forming the other contact point; this allows of considerable adjustment. At the point where the screw comes in contact with the spring there is a platinum rivet. When an operator is not always in attendance, this code-signaling attachment has some advantages over the drop as a signal interpreter, in that it permits the code signals to be heard from a distance. Of course, the addition of spring contacts to the drop armature tends to complicate the structure and perhaps to cut down the sensitiveness of the drop, which are offsetting disadvantages.

[Ill.u.s.tration: Fig. 261. Combined Drop and Ringer]

For really long lines, this code signaling by means of the drop is best provided for by employing a combined drop and ringer, although in this case whatever advantages are secured by the mechanical restoration of the shutter upon plugging in are lost. Such a device as manufactured by the Dean Electric Company is shown in Fig. 261. In this the ordinary polarized ringer is used, but in addition the tapper rod carries a latch which, when vibrated by the ringing of the bell, releases a shutter and causes it to fall, thus giving a visual as well as an audible signal.

_Electrical_. Coming now to the electrical restoration of drop shutters, reference is made to Fig. 262, which shows in side section the electrical restoring drop employed by the Bell companies and manufactured by the Western Electric Company. In this the coil _1_ is a line coil, and it operates on the armature _2_ to raise the latch lever _3_ in just the same manner as in the ordinary tubular drop.

The latch lever _3_ acts, however, to release another armature _4_ instead of a shutter. This armature _4_ is pivoted at its lower end at the opposite end of the device from the armature _2_ and, by falling outwardly when released, it serves to raise the light shutter _5_. The restoring coil of this device is shown at _6_, and when energized it attracts the armature _4_ so as to pull it back under the catch of the latch lever _3_ and also so as to allow the shutter _5_ to fall into its normal position. The method of closing the restoring circuit is by placing coil _6_ in circuit with a local battery and with a pair of contacts in the jack, which latter contacts are normally open but are bridged across by the plug when it enters the jack, thus energizing the restoring coil and restoring the shutter.

[Ill.u.s.tration: Fig. 262. Electrically Restored Drop]

A perspective view of this Western Electric electrical restoring drop is shown in Fig. 263, a more complete mention being made of this feature under the discussion of magneto multiple switchboards, wherein it found its chief use. It is mentioned here to round out the methods that have been employed for accomplis.h.i.+ng the automatic restoration of shutters by the insertion of the plug.

[Ill.u.s.tration: Fig. 263. Electrically Restored Drop]

Switchboard Plugs. A switchboard plug such as is commonly used in simple magneto switchboards is shown in Fig. 264 and also in Fig. 235.

The tip contact is usually of bra.s.s and is connected to a slender steel rod which runs through the center of the plug and terminates near the rear end of the plug in a connector for the tip conductor of the cord. This central core of steel is carefully insulated from the outer sh.e.l.l of the plug by means of hard rubber bus.h.i.+ngs, the parts being forced tightly together. The outer sh.e.l.l, of course, forms the other conductor of the plug, called the sleeve contact. A handle of tough fiber tubing is fitted over the rear end of the plug and this also serves to close the opening formed by cutting away a portion of the plug sh.e.l.l, thus exposing the connector for the tip conductor.

[Ill.u.s.tration: Fig. 264. Switchboard Plug]

_Cord Attachment._ The rear end of the plug sh.e.l.l is usually bored out just about the size of the outer covering of the switchboard cord, and it is provided with a coa.r.s.e internal screw thread, as shown. The cord is attached by s.c.r.e.w.i.n.g it tightly into this screw-threaded chamber, the screw threads in the bra.s.s being sufficiently coa.r.s.e and of sufficiently small internal diameter to afford a very secure mechanical connection between the outer braiding of the cord and the plug. The connection between the tip conductor of the cord and the tip of the plug is made by a small machine screw connection as shown, while the connection between the sleeve conductor of the plug and the sleeve conductor of the cord is made by bending back the latter over the outer braiding of the cord before it is screwed into the shank of the plug. This results in the close electrical contact between the sleeve conductor of the cord and the inner metal surface of the shank of the plug.

Switchboard Cords. A great deal of ingenuity has been exerted toward the end of producing a reliable and durable switchboard cord. While great improvement has resulted, the fact remains that the cords of manual switchboards are today probably the most troublesome element, and they need constant attention and repairs. While no two manufacturers build their cords exactly alike, descriptions of a few commonly used and successful cords may be here given.

_Concentric Conductors._ In one the core is made from a double strand of strong lock st.i.tch twine, over which is placed a linen braid. Then the tip conductor, which is of stranded copper tinsel, is braided on.

This is then covered with two layers of tussah silk, laid in reverse wrappings, then there is a heavy cotton braid, and over the latter a linen braid. The sleeve conductor, which is also of copper tinsel, is then braided over the structure so formed, after which two reverse wrappings of tussah silk are served on, and this is covered by a cotton braid and this in turn by a heavy linen or polished cotton braid. The plug end of the cord is reinforced for a length of from 12 to 18 inches by another braiding of linen or polished cotton, and the whole cord is treated with melted beeswax to make it moisture-proof and durable.