Visible Signals

Electromagnetic Signal. Practical visual signals are of two general kinds: electromagnetic devices for moving a target or pointer, and incandescent lamps. The earliest and most widely used visible signal in telephone practice was the annunciator, having a shutter adapted to fall when the magnet is energized. Fig. 22 is such a signal. Shutter 1 is held by the catch 2 from dropping to the right by its own gravity. The name "gravity-drop" is thus obvious. Current energizing the core attracts the armature 3 , lifts the catch 2 , and the shutter falls. A simple modification of the gravity-drop produces the visible signal shown in Fig. 23. Energizing the core lifts a target so as to render it visible through an opening in the plate 1 . A contrast of color between the plate and the target heightens the effect. IMG:384091999325875926_fig022_t.gif.png:Illustration_ Fig. 22. Gravity-Drop Fig. 22. Gravity-Drop View full size illustration. The gravity-drop is principally adapted to the magneto-bell system of signaling, where an alternating current is sent over the line to a central office by the operation of a bell crank at the subscriber's station, this current, lasting only as long as the crank is turned, energizes the drop, which may be restored by hand or otherwise and will remain latched. The visible signal is better adapted to lines in which the signaling is done by means of direct current, as, for example, in systems where the removal of the receiver from the hook at the subscriber's station closes the line circuit, causing current to flow through the winding of the visible signal and so displaying it until the receiver has been hung upon the hook or the circuit opened by some operation at the central office. Visible signals of the magnetic type of Fig. 23 have been widely used in connection with common-battery systems, both for line signals and for supervisory purposes, indicating the state and the progre of the connection and conversation. IMG:384091999325875926_fig023_t.gif.png:Illustration_ Fig. 23. Electromagnetic Visible Signal Fig. 23. Electromagnetic Visible Signal View full size illustration. IMG:384091999325875926_fig024_t.gif.png:Illustration_ Fig. 24. Lamp Signal and Lens Fig. 24. Lamp Signal and Lens View full size illustration. Electric-Lamp Signal. Incandescent electric lamps appeared in telephony as a considerable element about 1890. They are better than either form of mechanical visible signals because of three principal qualities: simplicity and ease of restoring them to normal as compared with drops; their compactne ; and their greater prominence when displayed. Of the latter quality, one may say that they are more insistent , as they give out light instead of reflecting it, as do all other visible signals. In its best form, the lamp signal is mounted behind a hemispherical lens, either slightly clouded or cut in facets. This lens serves to distribute the rays of light from the lamp, with the result that the signal may be seen from a wide angle with the axis of the lens, as shown in Fig. 24. This is of particular advantage in connection with manual-switchboard connecting cords, as it enables the signals to be mounted close to and even among the cords, their great visible prominence when shining saving them from being hidden. The influence of the lamp signal was one of the potent ones in the development of the type of multiple switchboard which is now universal as the mechanism of large manual exchanges. The first large trial of such an equipment was in 1896 in Worcester, Ma . No large and succe ful multiple switchboard with any other type of signal has been built since that time. Any electric signal has upper and lower limits of current between which it is to be actuated. It must receive current enough to operate but not enough to become damaged by overheating. The magnetic types of visible signals have a wider range between these limits than have lamp signals. If current in a lamp is too little, its filament either will not glow at all or merely at a dull red, insufficient for a proper signal. If the current is too great, the filament is heated beyond its strength and parts at the weakest place. This range between current limits in magnetic visible signals is great enough to enable them to be used direct in telephone lines, the operating current through the line and signal in series with a fixed voltage at the central office being not harmfully great when the entire line resistance is shunted out at or near the central office. The increase of current may be as great as ten times without damage to the winding of such a signal. In lamps, the safe margin is much le . The current which just gives a sufficient lighting of the signal may be about doubled with safety to the filament of the lamp. Consequently it is not feasible to place the lamp directly in series with long exposed lines. A short circuit of such a line near the central office will burn it out. IMG:384091999325875926_fig025_t.gif.png:Illustration_ Fig. 25. Lamp Signal Controlled by Relay Fig. 25. Lamp Signal Controlled by Relay View full size illustration. The qualities of electromagnets and lamps in these respects are used to advantage by the lamp signal arrangement shown in Fig. 25. A relay is in series with the line and provides a large range of sensibility. It is able to carry any current the central-office current source can pa through it. The local circuit of the relay includes the lamp. Energizing the relay lights the lamp, and the reverse; the lamp is thus isolated from danger and receives the current best adapted to its needs. All lines are not long and when enclosed in cable or in well-insulated interior wire, may be only remotely in danger of being short-circuited. Such conditions exist in private-branch exchanges, which are groups of telephones, usually local to limited premises, connected to a switchboard on those premises. Such a situation permits the omi ion of the line relay, the lamp being directly in the line. Fig. 26 shows the extreme simplicity of the arrangement, containing no moving parts or costly elements. Lamps for such service have improved greatly since the demand began to grow. The small bulk permitted by the need of compactne , the high filament resistance required for simplicity of the general power scheme of the system, and the need of considerable sturdine in the completed thing have made the task a hard one. The practical result, however, is a signal lamp which is highly satisfactory. IMG:384091999325875926_fig026_t.gif.png:Illustration_ Fig. 26. Lamp Signal Directly in Line Fig. 26. Lamp Signal Directly in Line View full size illustration. IMG:384091999325875926_fig027_t.gif.png:Illustration_ Fig. 27. Lamp Signal and Ballast Fig. 27. Lamp Signal and Ballast View full size illustration. The nature of carbon and certain earths being that their conductivity rises with the temperature and that of metals being that their conductivity falls with the temperature, has enabled the Nernst lamp to be succe ful. The same relation of properties has enabled incandescent-lamp signals to be connected direct to lines without relays, but compensated against too great a current by causing the resistance in series with the lamp to be increased inversely as the resistance of the filament. Employment of a "ballast" resistance in this way is referred to in Chapter XI. In Fig. 27 is shown its relation to a signal lamp directly in the line. 1 is the carbon-filament lamp; 2 is the ballast. The latter's conductor is fine iron wire in a vacuum. The resistance of the lamp falls as that of the ballast rises. Within certain limits, these changes balance each other, widening the range of allowable change in the total resistance of the line. ToC cHAPTER IV