Dial Telegraphs.— Variously known as dial, A, B, C, and pointer telegraph system; due to Breguet, Kramer, Frischen, Wheatstone and Siemens-Halske. These systems employ a dial carrying on an outer circle the letters of the alphabet and on an inner circle figures and punctuation marks. A pointer operated by suitable mechanism within a case moves like the minute hand of a clock around the dial in response to pulsations of electricity from a sending instrument. The sending in strument has a similar dial and is equipped with a key which is movable around the face of the dial. As the key is thus moved it opens or closes a circuit, or it causes the movement of a magnet before coils of wire within the case, that transmits pulsations of electricity over the line, which in turn actuate, by means of an electromagnet, an escape wheel that moves the pointer as stated. The operator moves the transmitting key uniformly around the dial. As he does so the pointer on the receiving instru ment moves a corresponding distance. When the key arrives at a desired letter the operator pauses. The receiving operator or attendant notes the letter; the sender moves his key to the next desired letter, and so on, in this man ner spelling out his message. This system is slow, but it possesses the advantage of requir ing little nr no skill on the part of the user for its operation. Hence, especially before the days of the telephone, it was in favor as a means of communication between police and fire headquarters, on railroads, etc. As else where stated telegraphs of this order are still in use in other countries.
Military and Naval Telegraph.— Heliog raphy is quite extensively employed by the army and navy of this and other countries. The distance covered between any two sta tions is about 25 miles. Signals are trans mitted by reflections of the rays of the sun, the duration of the reflections being made to correspond to dots and dashes of the Morse or any other prearranged code. One form of heliograph consists of a mirror freely mov able in a socket and supported on the upper end of a tripod. The reflections are thrown in the direction of the distant station and signals are sent by interposing the hand or a screen before the mirror. When the sun is not in a suitable position to admit of its direct reflection in a desired direction, two mirrors are employed, one facing the sun, which re flects the sun's rays upon the mirror facing the distant station. .
Flag-signaling, termed wig-wagging and flash signaling by lanterns and search lights, also by horns, whistles and sirens are em ployed by the various armies and navies of the world. Also Morse telegraphy and wireless telegraphy (q.v.). Torches at night take the place of the flag in signaling. In flag and torch signaling, as well as in heliography and lantern signaling the Continental Morse alphabet is now used. Arbitrary characters of this general type have long been used in needle telegraph systems in which a deflection to the left is represented by the figure alp; a deflection to the right by the figure Thus A would be represented by in one such alphabet. or two deflections to the right.
In wigwagging or *torch signaling the flag man faces exactly toward the distant station; staff is vertical in front of centre of body, but at height of waist. When the Morse alphabet is employed, a dot is represented by a motion of flag or torch to right; the dash, a motion to the left; the space by a °front* motion. See SEMAPHORE Printing Telegraphy, Stock Tickers.— Printing telegraphy relates to those telegraph systems in which telegrams are printed as re ceived on strips or sheets of paper. Generally speaking, printing telegraph systems of the stock ticker type depend for their operation upon the synchronous rotation of a wheel or cylinder at a receiving station with a type wheel at a receiving station. If, for example, two wheels of equal size having on their peripheries type letters of the alphabet are placed side by side and are caused to rotate, it is obvious that it .they start with a similar letter at a given point, each wheel will continue to present a similar letter at the given point as long as the wheels rotate at equal speeds. It is, however, a difficult matter to obtain a con tinuous synchronous rotation of two or more such wheels or cylinders, especially when they are revolving at a high rate of speed, unless their movements •are under control of some master wheel or transmitter. Consequently, in electrical printing telegraphy, controlling de vices of this nature are employed. For in stance, in the well-known (ticker" systems, the type-wheels of the tickers in the various offices are placed under control of a transmitter which maintains them in synchronism by a ustep-by step° movement, so called. In certain other printing telegraph systems, such as the Hughes, largely used in Europe, the uniform rotation of the transmitting and receiving wheels is maintained by a nearly synchronous rotation of the motors at each end of the circuit, and, in addition, by a 'correcting' device applied to the See Synchronous Multiplex Telegraph in this article.
A theoretical diagram of a simple "step-by step') printing telegraph system is shown in Fig. 16. The transmitting apparatus consists of a long cylinder T. A metal segmental W is rigidly mounted on the same shaft as the cylinder. An electric motor is mounted by friction bearings on the same shaft. The ob ject of using the friction bearings is to permit stopping the cylinder instantly, without stopping the motor. Cylinder T carries a set of blunt pins, p, p, projecting in a spiral row from its surface. A keyboard is placed above the cylinder, with the keys directly over the pins as indicated; one key for each pin. When a particular key is depressed, its spur, or catch, moves into the path of a corresponding phi, thereby instantly arresting the cylinder. The wheel W is insulated in two parts or segments as indicated by the zig-zag line i. As the wheel rotates, portions of the respective segments (s, s') alternately pass under brush B, with the result that momentary currents pass al ternately from positive and negative batteries b, b', giving the equivalent of an alternating current on the line.