Component Parts

Before considering further the operation of manual switchboards it will be well to refer briefly to the component pieces of apparatus which go to make up a switchboard. Line Signal. The line signal in magneto switchboards is practically always in the form of an electromagnetic annunciator or drop. It consists in an electromagnet adapted to be included in the line circuit, its armature controlling a latch, which serves to hold the drop or shutter or target in its raised position when the magnet is not energized, and to release the drop or shutter or target so as to permit the display of the signal when the magnet is energized. The symbolic representation of such an electromagnetic drop is shown in Fig. 233. IMG:384091999325875926_fig233_t.gif.png:Illustration_ Fig. 233. Drop Symbol Fig. 233. Drop Symbol View full size illustration. Jacks and Plugs. Each line is also provided with a connection terminal in the form of a switch socket. This a umes many forms, but always consists in a cylindrical opening behind which are arranged one or more spring contacts. The opening forms a receptacle for plugs which have one or more metallic terminals for the conductors in the flexible cord in which the plug terminates. The arrangement is such that when a plug is inserted into a jack the contacts on the plug will register with certain of the contacts in the jack and thus continue the line conductors, which terminate in the jack contacts, to the cord conductors, which terminate in the plug contacts. Usually also when a plug is inserted certain of the spring contacts in the jack are made to engage with or disengage other contacts in the jack so as to make or break auxiliary circuits. IMG:384091999325875926_fig234_t.gif.png:Illustration_ Fig. 234. Spring Jack Fig. 234. Spring Jack View full size illustration. A simple form of spring jack is shown in section in Fig. 234. In Fig. 235 is shown a sectional view of a plug adapted to co-operate with the jack of Fig. 234. In Fig. 236 the plug is shown inserted into the jack. The cylindrical portion of the jack is commonly called the sleeve or thimble and it usually forms one of the main terminals of the jack; the spring, forming the other principal terminal, is called the tip spring , since it engages the tip of the plug. The tip spring usually rests on another contact which may be termed the anvil . When the plug is inserted into the jack as shown in Fig. 236, the tip spring is raised from contact with this anvil and thus breaks the circuit leading through it. It will be understood that spring jacks are not limited to three contacts such as shown in these figures nor are plugs limited to two contacts. Sometimes the plugs have three, and even more, contacts, and frequently the jacks corresponding to such plugs have not only a contact spring adapted to register with each of the contacts of the plug, but several other auxiliary contacts also, which will be made or broken according to whether the plug is inserted or withdrawn from the jack. Symbolic representations of plugs and jacks are shown in Fig. 237. These are employed in diagrammatic representations of circuits and are supposed to represent the e ential elements of the plugs and jacks in such a way as to be suggestive of their operation. It will be understood that such symbols may be greatly modified to expre the various peculiarities of the plugs and jacks which they represent. IMG:384091999325875926_fig235_t.gif.png:Illustration_ Fig. 235. Plug Fig. 235. Plug View full size illustration. IMG:384091999325875926_fig236_t.gif.png:Illustration_ Fig. 236. Plug and Jack Fig. 236. Plug and Jack View full size illustration. IMG:384091999325875926_fig237_t.gif.png:Illustration_ Fig. 237. Jack and Plug Symbols Fig. 237. Jack and Plug Symbols View full size illustration. Keys . Other important elements of manual switchboards are ringing and listening keys. These are the devices by means of which the operator may switch the central-office generator or her telephone set into or out of the circuit of the connected lines. The details of a simple ringing and listening key are shown in Fig. 238. This consists of two groups of springs, one of four and one of six, the springs in each group being insulated from each other at their points of mounting. Two of these springs 1 and 2 in one group—the ringing group—are longer than the others, and act as movable levers engaging the inner pair of springs 3 and 4 when in their normal positions, and the outer pair 5 and 6 when forced into their alternate positions. Movement is imparted to these springs by the action of a cam which is mounted on a lever, manipulated by the operator. When this lever is moved in one direction the cam pre es the two springs 1 and 2 apart, thus causing them to disengage the springs 3 and 4 and to engage the springs 5 and 6 . IMG:384091999325875926_fig238_t.gif.png:Illustration_ Fig. 238. Ringing and Listening Key Fig. 238. Ringing and Listening Key View full size illustration. The springs of the other group constitute the switching element of the listening key and are very similar in their action to those of the ringing key, differing in the fact that they have no inner pair of springs such as 3 and 4 . The two long springs 7 and 8 , therefore, normally do not rest against anything, but when the key lever is pre ed, so as to force the cam between them, they are made to engage the two outer springs 9 and 10 . IMG:384091999325875926_fig239_t.gif.png:Illustration_ Fig. 239. Ringing-and Listening-Key Symbols Fig. 239. Ringing-and Listening-Key Symbols View full size illustration. The design and construction of ringing and listening keys a ume many different forms. In general, however, they are adapted to do exactly the same sort of switching operations as that of which the device of Fig. 238 is capable. Easily understood symbols of ringing and listening keys are shown in Fig. 239; the cam member which operates on the two long springs is usually omitted for ease of illustration. It will be understood in considering these symbols, therefore, that the two long curved springs usually rest against a pair of inner contacts in case of the ringing key or against nothing at all in case of the listening key, and that when the key is operated the two springs are a umed to be spread apart so as to engage the outer pair of contacts with which they are respectively normally disconnected. Line and Cord Equipments. The parts of the switchboard that are individual to the subscriber's line are termed the line equipment; this, in the case of a magneto switchboard, consists of the line drop and the jack together with the a ociated wiring nece ary to connect them properly in the line circuit. The parts of the switchboard that are a ociated with a connecting link—consisting of a pair of plugs and a ociated cords with their ringing and listening keys and clearing-out drop—are referred to as a cord equipment . The circuit of a complete pair of cords and plugs with their a ociated apparatus is called a cord circuit . In order that there may be a number of simultaneous connections between different pairs of lines terminating in a switchboard, a number of cord circuits are provided, this number depending on the amount of traffic at the busiest time of the day. Operator's Equipment. A part of the equipment that is not individual to the lines or to the cord circuits, but which may, as occasion requires, be a ociated with any of them is called the operator's equipment . This consists of the operator's transmitter and receiver, induction coil, and battery connections together with the wiring and other a ociated parts nece ary to co-ordinate them with the rest of the apparatus. Still another part of the equipment that is not individual to the lines nor to the cord circuits is the calling-current generator. This may be common to the entire office or a separate one may be provided for each operator's position.