313,964. Standard Telephones & Cables, Ltd., (Assignees of Keith, C R., and Peterson, E.). June 20, 1928, [Convention date]. Inductive effects, minimizing; two-way repeating systems. - In electric signalling systems, disturbances due to cross-talk, line noise, atmospherics and the like are minimized by the introduction into the circuits of semi-directional conducting elements having non-linear currentvoltage characteristics so as to discriminate against weak disturbing currents and thereby give a silent background during intervals between signals. The invention is described mainly in connection with telephony but may be applied to wireless and wired-wireless systems. The elements consist of crystal, metallic oxide or other rectifiers having a relatively high resistance to currents of small amplitudes, associated with the line by transformers so that they are effectively in series with the line. In the simplest arrangement shown in Fig. 1, the rectifier 4 having a current-voltage curve as shown in Fig. 1<A>, is coupled to a telephone line L by a transformer 1 which may be of the step-up type to ensure that the voltage induced in winding 3 by speech currents of voltage b, is sufficient to operate the rectifier on the low resistance portion of its characteristic. For weak disturbing currents, of voltage a, the effective resistance of the line is increased due to the relatively high resistance of the rectifier 4. In the arrangement shown in Fig. 2, two opposed rectifiers 14, 15 are used in conjunction with a centre-tapped transformer winding 13 so that the combined rectifying element has a symmetrical characteristic, Fig. 2<A>, thereby avoiding the distortion arising from the use of a single element. The winding in the rectifier circuit may be coupled to balancing coils in the line. In the arrangement shown in Fig. 4, a rectifier 34, resistance 35 and biasing battery 36 are used in conjunction with transformers 31, 37. The rectifier has a characteristic h, Fig. 4<A>, and the resistance is of a value equal to that of the rectifier for negative current, its linear characteristic being shown at i. The windings 38, 39 of transformer 37 are opposed so that the resultant current is indicated by the line k. The arrangement is biassed to bring the horizontal portion of k somewhat over to the positive side and entirely prevents the transmission of weak disturbing currents. The primary winding 32 may be split and each half coupled to a circuit such as 33 - - 39 but arranged so that one circuit transmits the positive half cycles and the other the negative half cycles of speech waves. In the modification shown in Fig. 7, a discriminator 12 - - 16 similar to that shown in Fig. 2 is inserted in the line. A portion of line current is tapped off through a resistance 4 and transformer 1 and after amplification by a valve 5 and full wave rectification by rectifiers 10, 11, is utilized to bias the elements 12, 13. For weak disturbing currents, the biassing voltage is small and the characteristic of the discriminator is substantially as shown in Fig. 2<A>, but during conversation, a larger biassing voltage is available and the characteristic takes the form shown in Fig. 10 in which the region of non-transmission is eliminated. The amplifying valve may be omitted. The application of the invention to two-way repeaters (Fig. 6 not shown) is also described. The Specification as open to inspection under Sect. 91 (3) (a) comprises rectifiers inserted directly in the line, a shunt type discriminator using thermionic valve rectifiers, and arrangements using series and shunt discrimination in combination. The shunt type discriminator comprises a diode rectifier 204, Fig 10 (Cancelled), connected across the line by a, transformer 201, the emission being maintained constant by a ballast lamp 207 in the filament circuit, and the plate being positively biassed by battery 209. The rectifier has the characteristic shown in Fig. 10<A> (Cancelled), so that it provides a low resistance path for currents due to small disturbing voltages e, while for larger voltage speech waves f, one half cycle is blocked and the rectifier works above saturation point for the other half cycle. In the shunt type discriminator shown in Fig. 15 (Cancelled) a pair of balanced triode rectifiers 77, 78 are connected across the line by a transformer 84. A portion of line current is tapped off through transformer 61, and after amplification by valve 64 and full wave rectification by valves 67, 6S is utilized, to bias the plates of the rectifiers 77, 78. The biassing voltage due to disturbing currents is small and the characteristic of the discriminator is of the form a, Fig. 15A (Cancelled), so that the discriminator provides a low resistance, shunt for such currents. During conversation, the biassing voltage is higher and the characteristic takes the form b so that the resistance of the discriminator is large and substantially all the speech energy is passed on. The discriminator 77, 78 may be used without the biassing arrangement of valves 64, 67, 68 but, due to the form a, Fig. 15A (Cancelled) of its characteristic, there would be some loss or distortion of speech signals. The use of discriminators is also described in connection with repeating coils for phantom-circuit systems. This subject-matter does not appear in the Specification as accepted.