Classes of Telegraphs: Needle to the method of signalling- employed, telegraphs are distinguished as needle, dial, print ing, and copying telegraphs. The needle telegraph is made to convey sig. nals by the deflection of a magnetic needle through the medium of an elec tric current caused to flow through a coil of insulated copper wire within which it is suspended.
IT7Icaisione ana' Cooke's Needle 22 and 23 exhibit, respectively, front and rear views of the simple needle telegraph of 'Wheat stone and Cooke. _4 (fig. 23) is the multiplier surrounding- the needle, and consisting of a coil of fine silk-covered copper wire wound round a thin frame of brass. An exterior needle (fig. 22) which serves as the indicator for communicating- the signals is carried on the same axis as the needle of the multiplier; to neutralize the influence of the earth's magnetism, the poles of the two needles, however, are placed in reverse position relative to each other. Figure 24 is a diagram exhibiting two stations connected by this system of communication. G, G' arc the signal-key handles; A', A.7, the multipliers; g', the line wire. The key-handle G, projecting from the exterior of the enclosing frame, is mounted on the same axis with a disc of wood in the interior, upon the periphery of which are placed seven con ducting pieces of brass separated from one another by interposed strips of ivory (pi. 57, fig. 24, Certain of these brass pieces are connected with one another by strips of brass—for example, a with b and c, f with a', e with g. Four contact springs, .1-, y, and n, press against the edge of the disc, of which the first two are connected with the poles of the battery, m with the ground plate, and n with one terminal of the multiplier. When the two handles, G and G', are placed vertically, the batteries are cut out from the line. When the handle G' is placed to the right, as shown in the Figure, the current passes from -4- to — pole in one direction. Turning the handle to the opposite side sends the current in the opposite direction through the apparatus, and consequently produces a deflection of the needles to the opposite side. The alphabet is formed from various prede termined combinations of the needle deflections.
Thomson' s Cable Telegraph. —The needle teleg,raph was employed in working the submarine line between Ireland and America, Professor (now Sir William) Thomson haying- devised an apparatus for the purpose cor responding in principle with the reflecting, or mirror, galvanometer used by Gauss and Weber in 1833 (p. 346). It consists of a slender bar of steel suspended from a cocoon-thread and carrying at its point of suspension a very delicate mirror. The needle passes within a coil of fine copper wire. At the distance of about 3 feet a lamp contained within a sliet-inetal box throws upon the mirror a slender beam of light through a slot, provided for the purpose, in thc box. The mirror reflects this beam upon the grad uated scale. Any deflection of the needle, however slight, in consequence
of the passage of the current through the line, is thereupon reproduced and greatly magnified by the movement of the band of light on the scale. Fig ure 2 (p. 5S) represents the appearance and operation of the ingenious siphon recorder devised by Sir William Thomson, and designed actually to delineate on paper the irregular movements of the reflecting galvan ometer above referred to.
Telegraphs, which are nothing more than a combination of two needle telegraphs of the kind just described, afford greater faciJity in the transmission of signals, but require two line wires, and are now but rarely used.
Dial —For the utilization of magneto-electric currents, the several forms of the " dial telegraphs "—so called to disting,tiish them from the above-described needle telegraphs operated by currents of similar ori g-,in—are found verv serviceable. In this modification an index, by suit able mechanical appliances, is made to revolve over a set of letters or other symbols placed around the circumference of a dial. By other appropriate means this pointer is caused to stop opposite any desired character, and thus a message is spelled out letter by letter.
The Siemens Dial Telegraph ( 57, jigs. one of the most con venient and valuable of its class, has been in use for many years on the government telegraph-lines of several European countries (Russia, I3avaria), and on the municipal lines of some of the principal European cities. The following description of this system, abridged from Prescott, explains its construction and operation. It consists of a battery of permanent magnets, between the poles of which revolves a coil of insulated wire wound upon a soft-iron armature. This coil develops alternate positive and negative cur rents, which traverse the line and pass through the coils of an electro magnet at the receiving station, causing its armature to vibrate, an escape wheel and a pointer. Figure 2S (pi. 57) shows this apparatus with its wooden case len-loved. In operating, the handle, PI, is turned from letter to letter, these being marked on a horizontal dial-plate, J, stopping always against the tooth opposite the letter to be indicated. The metallic dial plate, J, having ratchet-shaped teeth, i, upon its rim, is fixed upon the top of the compound magnet, which is formed of a number of permanent mag nets, G, G', screwed to an upright soft-iron plate. Between the poles of this system of permanent magnets is placed a cylinder of soft iron, E, E, which serves as an armatnre. The interior of the indicator is shown in per spective in Figure 29. .The call-bell or alarm nsed with this instrument is seen at the top of the case in Figure 27. In Figure 25, which is one of Wheatstone and Cooke's dial telegraphs, A represents the key of the instru ment in the despatching station, and B the indicating apparatus at the receiving station.