Morse Circuit Repeaters.—A very simple form of repeater con sisting of only two relays may be used in single-current open circuit working. Each of the two lines is connected through the contacts and armature lever of one relay to the electromagnet coils of the other relay. When a pulse of current representing a signal is received over either line the electromagnet Of one relay is energized and its armature lever, in responding, applies a bat tery or other source of current to the other line, thus repeating the signal into the latter.
For closed-circuit working a more complex arrangement is necessary in order to avoid the tendency of each relay to repeat signals back into the line from which they were received. This interference is overcome by interlocking electromagnetic devices.
To facilitate adjustments many repeaters employ additional relays or transmitters to control the circuit into which signals are being repeated, thus relieving the receiving relay in each case of any duty other than the control of such a transmitter.
Duplex Repeaters.—The repeating elements of duplex repeat ers are very similar to those of open circuit Morse repeaters, usually comprising a receiving relay for each of the two line sections. This relay, either directly or by means of a transmitter which it controls, re-transmits into the other line section the signals by which it is operated. An artificial line must be pro vided for each of the two sections of line connected to the re peater, and each relay must be arranged differentially or in accordance with the bridge method so as to respond only to incoming signals. Various auxiliary devices are also required to permit observation of the passing signals in both directions by an attendant and to facilitate necessary readjustments of the appa ratus.
Regenerative Repeaters.—Recent years have witnessed much development of regenerative repeaters by which the signals are not only repeated with renewed power but are also corrected and reformed, so that they leave the repeater with all the character istics of the original signals, or, in some cases, with even better qualities than in the initial transmission. The general principle upon which regeneration is based is to select from each received signal a small portion—usually near the centre of the time inter val occupied by the signal unit—which is most likely to be of full strength and free from imperfections. By using this fraction
of the signal to operate a relay and other mechanisms which con trol the pulses to be applied to the outgoing line, the re-trans mitted signal can be made practically perfect.
Composite Method.—In the composite arrangement (fig. 2o) two ground return telegraph circuits are obtained from the two wires forming a metallic return telephone circuit. This is accom plished by taking advantage of the facts that a condenser offers little impedance to the alternating currents of relatively high frequency which operate the telephone, but is an almost impass able barrier to the much slower changes of current used in telegraphy, and that an inductance in the form of an electro magnetic coil can be so constructed that it will oppose greatly the flow of the telephone currents but will readily pass the tele graph currents. By forming a network of such condensers and inductances an electrical filter is obtained which guides the two classes of currents into their proper channels for the operation of the telegraph and telephone apparatus.
Simplex Method.—The simplex method of superposing permits only one ground return telegraph circuit to be operated over the two wires used to form a metallic return telephone circuit. In a common form of simplexed circuit (fig. 21), a repeating coil or transformer (see ELECTRICITY) is interposed at each terminal between the telephone apparatus and the line. The winding of this transformer, which is connected to the line wires, is tapped at its centre and this tap is connected to the telegraph apparatus. By this arrangement the telegraph currents are divided, one-half passing over each of the two line wires. These currents produce no effect upon the telephone apparatus because in dividing at the centre tap of the transformer winding the two equal parts must pass around the core in opposite directions and thus neutralize each other's magnetic effects. On the other hand, the alternating currents of the speech transmission so traverse the line windings of the transformer as to cause the maximum magnetic effect, and are thus repeated to the telephone apparatus.