Volume I Part 30 (2/2)
[Ill.u.s.tration: Fig. 265. Switchboard Cord]
_Steel Spiral Conductors._ In another cord that has found much favor the two conductors are formed mainly by two concentric spiral wrappings of steel wire, the conductivity being reinforced by adjacent braidings of tinsel. The structure of such a cord is well shown in Fig. 265. Beginning at the right, the different elements shown are, in the order named, a strand of lock st.i.tch twine, a linen braiding, into the strands of which are intermingled tinsel strands, the inner spiral steel wrapping, a braiding of tussah silk, a linen braiding, a loose tinsel braiding, the outer conductor of round spiral steel, a cotton braid, and an outside linen or polished cotton braid. The inner tinsel braiding and the inner spiral together form the tip conductor while the outer braiding and spiral together form the sleeve conductor. The cord is reinforced at the plug end for a length of about 14 inches by another braiding of linen. The tinsel used is, in each case, for the purpose of cutting down the resistance of the main steel conductor.
These wrappings of steel wire forming the tip and sleeve conductors respectively, have the advantage of affording great flexibility, and also of making it certain that whatever strain the cord is subjected to will fall on the insulated braiding rather than on the spiral steel which has in itself no power to resist tensile strains.
_Parallel Tinsel Conductors._ Another standard two-conductor switchboard cord is manufactured as follows: One conductor is of very heavy copper tinsel insulated with one wrapping of sea island cotton, which prevents broken ends of the tinsel or knots from piercing through and short-circuiting with the other conductor. Over this is placed one braid of tussah silk and an outer braid of cotton. This combines high insulation with considerable strength. The other conductor is of copper tinsel, not insulated, and this is laid parallel to the thrice insulated conductor already described. Around these two conductors is placed an armor of spring bra.s.s wire in spiral form, and over this a close, stout braid of glazed cotton. This like the others is reinforced by an extra braid at the plug end.
Ringing and Listening Keys. The general principles of the ringing key have already been referred to. Ringing keys are of two general types, one having horizontal springs and the other vertical.
[Ill.u.s.tration: Fig. 266. Horizontal-Spring Listening and Ringing Key]
_Horizontal Spring Type._ Various Bell operating companies have generally adhered to the horizontal spring type except in individual and four-party-line keys. The construction of a Western Electric Company horizontal spring key is shown in Fig. 266. In this particular key, as ill.u.s.trated, there are two cam levers operating upon three sets of springs. The cam lever at the left operates the ordinary ringing and listening set of springs according to whether it is pushed one way or the other. In ringing on single-party lines the cam lever at the left is the one to be used; while on two-party lines the lever at the left serves to ring the first party and the ringing key at the right the second party.
In order that the operator may have an indication as to which station on a two-party line she has called, a small target _1_ carried on a lever _2_ is provided. This target may display a black or a white field, according to which of its positions it occupies. The lever _2_ is connected by the links _3_ and _4_ with the two key levers and the target is thus moved into one position or the other, according to which lever was last thrown into ringing position.
It will be noticed that the springs are mounted horizontally and on edge. This on-edge feature has the advantage of permitting ready inspection of the contacts and of avoiding the liability of dust gathering between the contacts. As will be seen, at the lower end of each switch lever there is a roller of insulating material which serves as a wedge, when forced between the two long springs of any set, to force them apart and into engagement with their respective outer springs.
[Ill.u.s.tration: Fig. 267. Vertical-Spring Listening and Ringing Key]
_Vertical Spring Type._ The other type of ringing and listening key employing vertical springs is almost universally used by the various independent manufacturing companies. A good example of this is shown in Fig. 267, which shows partly in elevation and partly in section a double key of the Monarch Company. The operation of this is obvious from its mode of construction. The right-hand set of springs of the right-hand key in this cut are the springs of the listening key, while the left-hand set of the right-hand key are those of the calling-plug ringing key. The left-hand set of the left-hand key may be those of a ring-back key on the answering plug, while the right-hand set of the left-hand key may be for any special purpose. It is obvious that these groups of springs may be grouped in different combinations or omitted in part, as required. This same general form of key is also manufactured by the Kellogg Company and the Dean Company, that of the Kellogg Company being ill.u.s.trated in perspective, Fig. 268. The keys of this general type have the same advantages as those of the horizontal on-edge arrangement with respect to the gathering of dust, and while perhaps the contacts are not so readily get-at-able for inspection, yet they have the advantage of being somewhat more simple, and of taking up less horizontal s.p.a.ce on the key shelf.
[Ill.u.s.tration: Fig. 268. Vertical Listening and Ringing Key]
[Ill.u.s.tration: Fig. 269. Four-Party Listening and Ringing Key]
_Party-Line Ringing Keys._ For party-line ringing the key matter becomes somewhat more complicated. Usually the arrangement is such that in connection with each calling plug there are a number of keys, each arranged with respect to the circuits of the plug so as to send out the proper combination and direction of current, if the polarity system is used; or the proper frequency of current if the harmonic system is used; or the proper number of impulses if the step-by-step or broken-line system is used. The number of different kinds of arrangements and combinations is legion, and we will here ill.u.s.trate only an example of a four-party line ringing key adapted for harmonic ringing. A Kellogg party-line listening and ringing key is shown in Fig. 269. In this, besides the regular listening key, are shown four push-b.u.t.ton keys, each adapted, when depressed, to break the connection back of the key, and at the same time connect the proper calling generator with the calling plug.
_Self-Indicating Keys._ A complication that has given a good deal of trouble in the matter of party-line ringing is due to the fact that it is sometimes necessary to ring a second or a third time on a party-line connection, because the party called may not respond the first time. The operator is not always able to remember which one of the four keys a.s.sociated with the plug connected with the desired party she has pressed on the first occasion and, therefore, when it becomes necessary to ring again, she may ring the wrong party. This is provided for in a very ingenious way in the key shown in Fig. 269, by making the arrangement such that after a given key has been depressed to its full extent in ringing, and then released, it does not come quite back to its normal position but remains slightly depressed. This always serves as an indication to the operator, therefore, as to which key she depressed last, and in the case of a re-ring, she merely presses the key that is already down a little way. On the next call if she is required to press another one of the four keys, the one which remained down a slight distance on the last call will be released and the one that is fully depressed will be the one that remains down as an indication.
Such keys, where the key that was last used leaves an indication to that effect, are called _indicating_ ringing keys. In other forms the indication is given by causing the key lever to move a little target which remains exposed until some other key in the same set is moved.
The key shown in Fig. 266 is an example of this type.
NOTE. The matter of automatic ringing and other special forms of ringing will be referred to and discussed at their proper places in this work, but at this point they are not pertinent as they are not employed in simple switchboards.
Operator's Telephone Equipment. Little need be said concerning the matter of the operator's talking apparatus, _i.e._, the operator's transmitter and receiver, since as transmitters and receivers they are practically the same as those in ordinary use for other purposes. The watch-case receiver is nearly always employed for operators' purposes on account of its lightness and compactness. It is used in connection with a head band so as to be held continually at the operator's ear, allowing both of her hands to be free.
The transmitter used by operators does not in itself differ from the transmitters employed by subscribers, but the methods by which it is supported differ, two general practices being followed. One of these is to suspend the transmitter by flexible conducting cords so as to be adjustable in a vertical direction. A good ill.u.s.tration of this is given in Fig. 270. The other method, and one that is coming into more and more favor, is to mount the transmitter on a light bracket suspended by a flexible band from the neck of the operator, a breast plate being furnished so that the transmitter will rest on her breast and be at all times within proper position to receive her speech. To facilitate this, a long curved mouthpiece is commonly employed, as shown clearly in Fig. 47.
[Ill.u.s.tration: Fig. 270. Operator's Transmitter Suspension]
_Cut-in Jack._ It is common to terminate that portion of the apparatus which is worn on the operator's person--that is, the receiver only if the suspended type of transmitter is employed, and the receiver and transmitter if the breast plate type of transmitter is employed--in a plug, and a flexible cord connecting the plug terminates with the apparatus. The portions of the operator's talking circuit that are located permanently in the switchboard cabinet are in such cases terminated in a jack, called an operator's _cut-in jack_. This is usually mounted on the front rail of the switchboard cabinet just below the key shelf. Such a cut-in jack is shown in Fig. 271 and it is merely a specialized form of spring jack adapted to receive the short, stout plug in which the operator's transmitter, or transmitter and receiver, terminate. By this arrangement the operator is enabled readily to connect or disconnect her talking apparatus, which is worn on her person, whenever she comes to the board for work or leaves it at the end of her work. A complete operator's telephone set, or that portion that is carried on the person of the operator, together with the cut-in plug, is shown in Fig. 272.
[Ill.u.s.tration: Fig. 271. Operator's Cut-in Jack]
[Ill.u.s.tration: Fig. 272. Operator's Talking Set]
Circuits of Complete Switchboard. We may now discuss the circuits of a complete simple magneto switchboard. The one shown in Fig. 273 is typical. Before going into the details of this, it is well to inform the student that this general form of circuit representation is one that is commonly employed in showing the complete circuits of any switchboard. Ordinarily two subscribers' lines are shown, these connecting their respective subscribers' stations with two different line equipments at the central office. The jacks and signals of these line equipments are turned around so as to face each other, in order to clearly represent how the connection between them may be made by means of the cord circuit. The elements of the cord circuit are also spread out, so that the various parts occupy relative positions which they do not a.s.sume at all in practice. In other words it must be remembered that, in circuit diagrams, the relative positions of the parts are sacrificed in order to make clear the circuit connections.
However, this does not mean that it is often not possible to so locate the pieces of apparatus that they will in a certain way indicate relative positions, as may be seen in the case of the drop and jack in Fig. 273, the drop being shown immediately above the jack, which is the position in which these parts are located in practice.
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