The Wheatstone automatic receiver, or ink recorder, consists of a polarized relay (see Polar Duplex in this article), the armature lever of which is extended at a right angle at its upper end, and this extension at certain times is caused to impinge against a light rod pivoted at one end. On the end of this rod is a small circular disc, the lower portion of whose periphery is immersed in an ink well; an upper portion of its periphery is placed very near the stiff paper tape previously men tioned. The axle of this disc is given a slight tension away from the paper. When however, a current of positive polarity, designed to re cord a dot or dash on the paper, actuates the polarized relay, its extended armature lever presses against the axle of the disc causing it to deposit a mark on the moving paper tape. A negative current causes the withdrawal of the disc from the paper. The positive current in this system is termed a °marking) current; the negative current a °spacing° current, when it is operated by the °double current° or re versal of polarity method; to which further reference will be made in connection with Du plex Telegraphy, also in this article. The circular disc is kept in rotation by simple clocic work mechanism and thus constantly renews its supply of ink. The paper strip is drawn for ward at a desired rate by rollers operated by mechanism within the box.
The Wheatstone automatic telegraph may be worked duplex by using a differentially wound relay and the other necessary appa ratus of a duplex system. When worked as a duplex it gives a wire of moderate length, in which there are no very long submarine or underground cables, a capacity of from 200 to 350 words in each direction. On a 1,000-mile duplex circuit, such as from New York to Chicago, with one repeater station at tuffalo, a speed of about 125 words per minute in each direction is now obtainable.
Writing Telegraph Systems.— Writing or autothatic telegraph sysletns transmit and record facsimiles of letters or characters while they are being formed by the stylus or pen in the hand of the operator. The first to produce a writing telegraph system was probably Mr. A. E. Cowper, of England, who employed in the operation of his system the principle of the parallelogram of forces, whereby by cotn pounding the movements of a point in two di rections, the one at an angle to the other, the actual movement of the point is the resultant of the two movements. The Telautograph (q.v.) also operates on this principle. In the Cowper system the receiving pen depends for its movements upon variations of the magnet ism of two electromagnets placed at right angles to one another, which variations produce changes in their magnetic fields, to which changes an armature carrying the receiving pen is free to respond. The magnets are placed in
separate circuits. The variations in the mag netic strength of the magnets are caused hy vanations of the current strength in their cir cuits, which variations are brought about by means of a pencil which when moved by the operator in the act of writing is caused to switch resistance coils in and out of the re spective circuits.
Duplex Telegraphy.— This consists in the sending of two messages over one wire in opposite directions at the same time. Since on an ordinary Morse telegraph circuit it is not possible to send more than one message at a time over a wire, it is essential, in order to make duplex telegraphy possible, that the signals transmitted from the .sendmg station shall not interfere with the signals. to be re ceived at that station. The receonng instru ments must, therefore, be so constructed or so placed that while ready to respond to all signals from the distant station they shall not respond to signals from the near or home sta tion. These requirements are met in two ways in practice; one method being known as the °differential,* the other as the °bridge° method. The differential plan is used almost exclusively on land line duplex telegraphy; the bridge plan is utilized exclusively on long submarine cable telegraphy. The °differential* plan avails of the fact that if a soft iron bar or core, B Fig. 10, be wound with two coils of wire — oppositely wound as shown—a current from a battery b will flow in opposite directions around the iron bar, and as each current tends to set up opposite magnetic poles in the iron, one current will neutralize the other and no mag netic effects will be produced in the core. If, however, another battery or source of electro motive force should be placed in the circuit of wire 1 at its distant or right-hand end, a greater amount of current will flow in coil I than in coil 2, and in consequence the core will 'be magnetized to a degree depending on the difference in the strength of currents flowing in the respective coils; hence the term differential, and a relay wound in this way is termed a differential relay. The theory of the °bridge" plan is outlined in Fig. 11. The battery, and wires 1. and 2 remain as before, but the iron bar B is now connected between them and is wound with but one coil. a', b' are coils of resistance, termed the arms of the °bridge° (Wheatstone bridge). Assuming the resistance of wires a', b' and wires 1 and 2 to be equal, the electric pressure due to battery b, at the terminals of the bridge wire, will be equal and opposite, and hence no current will flow in the bridge wire.