Instruments.—The accompanying illustrations show several kinds of telegraph instruments commonly used on Morse circuits. Plate II., 2 and 3 are keys or transmitting devices, of the three types adapted respectively to closed circuit working as practised on American lines, and to the open circuit method used in Europe and elsewhere. Plate II., fig. 6 shows a modern form of Morse register or recorder; such instruments, however, are now used on comparatively few circuits. Instead of the register, the receiving instrument employed upon nearly all Morse circuits is the sounder as shown in Plate II., fig. 5. The intervals of time between the various clicks and the lengths of the clicks given out by the sounder enable a skilled operator to distinguish the dots, dashes and spaces of the signalling code and thus to read by sound the message being received. On circuits of such a character that the received currents are too weak to operate directly, the register or sounder, a relay (Plate II., fig. 4) may be used to receive the signals and, in responding, to control the application to the regis ter or sounder of suitable power from a local source.
The instruments described and illustrated above are for single current operation. Those used in double current working are somewhat different, the receiving instruments usually being fitted with a permanent magnet to polarize the armature, while the sending instruments have means for reversing the direction of the current instead of merely opening and closing the circuit.
Single Current IV orking.—The single current Morse system most used in Europe employs an open circuit arrangement, fig. 9, so designated because no current passes over the circuit while it is idle. Sending and receiving instruments are connected to the line at each station, however, and whenever the transmitting key at any station is depressed, a battery at that station is connected to the line and the resulting current causes the receiving instru ment at all other stations to respond. American single current Morse circuits are generally operated upon the closed circuit plan (fig. 15) under which current traverses the entire circuit at all times, when the line is idle as well as when a dot or a dash signal is being transmitted. The only periods during which no current flows are those corresponding to the spaces between the dots and dashes, this condition being effected by the opening of the trans mitting key at one of the stations. At such times the armature levers of the receiving instruments at all stations are released.
The reading of the received signals and the manipulation of the sending key by the operator are practically the same under both open and closed circuit arrangements. An advantage of the open circuit scheme is its economy in using electrical energy only while signals are actually being sent, but this is offset in many cases by the cost of providing at each station a battery or other equal to one dash. The space between words is twice one dash.
Fig. 8 shows the American and the International adaptations of the Morse code as used on land lines and on submarine cables. The cable code, adapted from the International, employs signals of dot duration only. These short signals are distinguished by their position on a tape; a short signal above the central line corresponds to a dot ; an equally short signal below, a dash. On land lines signals are usually received by sound on a Morse sounder; on submarine cables the Kelvin syphon recorder is used. The speed of manual working on land lines ranges from 20 to 4o words a minute, a fair average being 25 to 3o words a minute. On moderately long ocean cables, operated by hand on the syphon recorder system, somewhat similar speeds are obtained.
Various codes of abbreviations are applied to telegraphy, con sisting of single Morse letters and combinations of two or more letters that arbitrarily represent figures, words and phrases, to reduce the cost of telegraphing.
In double current operation the current pulses representing the source of power sufficient to operate the entire circuit. The closed circuit plan permits the entire power to be supplied at one or two stations, and this advantage generally outweighs the cost of supplying current to the circuit almost continuously, particu larly where many intermediate stations are connected. It is not uncommon to have 3o to 4o stations upon one circuit in America.
Double Current Working.—Double current Morse circuits (fig. I I), like those operated by the single current open circuit method, require at each station a source of power capable of operating the entire circuit The double current scheme has the areat advan Cage over any single current arrangement, however, of operating more reliably, and with less need of readjustments of apparatus, upon circuits of inferior transmission qualities, such as those due to excessive length of line, poor insulation, or the inclusion of long sections of cable.