Facsimile Systems.—Among the facsimile systems may be in cluded the writing telegraphs, such as the telautograph invented by Prof. Elisha Gray. (See fig. 17.) The transmitter for these systems consists of two rheostats which are varied through suit able linkages by the motion of a stylus. Each rheostat is con nected to a separate line wire which terminates through an electro magnet at the receiving end. The armatures of the magnets are connected by similar linkages to a recording pen. As the sending stylus is moved to form a letter the vertical components of the motion are caused to vary one of the rheostats, while horizontal components vary the other, changing the currents in the two line wires. In this way the attractive forces of the two receiving magnets are changed in proportion and the receiving pen is caused to duplicate the motion of the sending stylus.
The more modern facsimile systems are properly regarded as systems of telephotography. They are developments of earlier systems, such as Blakewell's and d'Alincourt's copying telegraphs and the Casilli pantelegraph. (See Plate I. fig. 4.) In these sys tems the picture is specially prepared for transmission by a photographic process and then exposed, a minute area at a time, to a transmitting device by means of which the line current is These line signals are received and recorded in code form on a moving paper tape. In the Wheatstone system marking impulses are transmitted for long and short intervals to form dots and dashes. An electromagnet in the receiver forces an inked wheel against a moving strip of paper, drawing an interrupted straight line composed of dots and dashes. This receiver is essentially a refinement of the early Morse recorders.
The recording systems commonly used on older types of ocean cables are developments of ideas proposed by Siemens and Halske, and Lord Kelvin. In these systems the dots and dashes are transmitted as impulses of equal duration but opposite in polarity, while spaces are intervals of no current. The recorder is a galvanometer with an ink syphon attached to the moving coil.
varied proportionately with the degree of shading in the exposed area. At the receiving end equivalent minute areas of a sensi tized paper are synchronously exposed to discolouration, the de gree being controlled by the variations in line current, thus dupli cating the contrasts of the original picture. Among the systems now in use may be mentioned those developed by the American Telephone and Telegraph Company, The Radio Corporation of America, and Siemens and Halske, together with the Bartlane, Belin and Ferreegraph systems.
Printing Telegraphs.—Many of the early printing telegraphs were developments of the step-by-step principle as incorporated in present-day stock quotation tickers. Printers operated by this principle are provided with a rotatable type-wheel upon which the characters are evenly spaced around the periphery. This type
wheel is provided with a stepping device, operated by an electro magnet and so arranged that each impulse received from the line will rotate the type-wheel one character. Assuming that the initial position and arrangement of the type-wheel are known, the sending operator may transmit a sufficient number of impulses to move any desired character into the printing position. When this has been done, an impulse of a different character is trans mitted which energizes a print magnet to make the impression. These systems are commonly provided with an automatic key board transmitter containing a motor-driven commutator which, in the interval between the depression of any two keys, will send the required number of impulses to move the type-wheel from its previous position to the character corresponding to the key next to be depressed. The most important improvement of this principle was contributed by Hughes, who materially speeded up the system by causing the sending and receiving apparatus to maintain constant synchronism. Line impulses were trans mitted only for the purpose of making an impression when the type-wheel reached the desired position. Other improvements were contributed by Phelps, Wright, Burry and Scott.
The majority of the modern printing systems make use of the Baudot or five-unit code. In these systems five impulses, any one of which may be either selecting or non-selecting, are transmitted for each character. With such a code it is possible to obtain 31 different combinations, 26 of which are assigned to the letters of the alphabet, leaving five for functions such as line feed, space, etc. (fig. i8). The five impulses making up the code are sent to the line, successively, by means of a rotating distributor or com mutator, and are distributed at the receiving end, by means of a similar device, to five receiving magnets. These magnets, through any one of several selecting mechanisms, determine which one of the characters of a typewriting machine is to be printed. In order that the first impulse may operate the first receiving magnet, etc., each magnet must be connected with the line while the corre sponding impulse is being transmitted, i.e., the sending and receiv one involves the use of five keys which must be set up in their various combinations by the operator; another employs a direct keyboard like a typewriter, each key acting upon five transmit ting keys to set up the combinations. The most common method uses a keyboard perforator which prepares a paper tape to be sent through an automatic transmitter. (See Plate I. fig. 3). The perforations in the tape actuate five key levers in the transmitter.