In order to decrease the loss of signalling energy in the line its inductance may be increased by artificial means known as load ing. This is especially successful with long submarine cables. The effect of the inductance is particularly to sustain the higher fre quencies. The velocity of propagation is decreased, and the signal, arriving slightly later, is received with much increased energy. Interference.—Telegraph operation is more or less affected by parasitic currents from various outside sources. These sources include: (I) earth potentials arising from natural causes (see MAGNETISM) ; (2) earth potentials arising from electric rail way systems; (3) induction from one telegraph circuit to another; and (4) induction from electric railway or power supply sys tems. The relative susceptiveness of the several forms of tele operation is as follows, expressed in order of decreasing susceptiveness: (i) submarine cable; (2) quadruplex; (3) multi plex; (4) duplex; and (5) single Morse. Because of the extensive use of multiplex telegraphy interference with circuits of that type is by far the most serious. Small amounts of potential from extraneous sources result in impairment of the maximum speed at which these circuits may be operated; large amounts render the circuits inoperative. With extremely high potentials persons operating the circuits may suffer bodily injury, or property dam age may result. To mitigate the interference from these parasitic currents various means may be applied to the 7.elegraph circuit
or to the source of the interference, if this be an electric railway or power supply system. In application to telegraph circuits these mitigative devices may take the form of impedances whose function is to filter out the extraneous current, or of trans formers or generators whose function is to interpose in the circuit a potential to counteract that from the outside source. In appli cation to electric railway or power systems, impedances inserted at appropriate points, selection of proper transformer connections, favourable operating methods or transpositions may often be advantageously employed to reduce the inductive influence of the interfering system. Sometimes, however, an increase in the physi cal separation between systems is the only practicable remedy.