317,760. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Aug. 19, 1929. [A Specification was laid open to inspection under Sect. 91 of the Acts, Aug. 22, 1929]. Repeating and amplifying systems.-Two-way repeaters comprise pairs of negative impedances coupled to a line by a network such as a hybrid coil, the impedances of a pair having different characteristics such that their use in combination prevents feed-back of current amplified by their action. In one arrangement, shown for phantom working in Fig. 6, the pairs of negative impedances A1, A<1>1 - - A3, A<1>3 have characteristics, explained subsequently with reference to Figs. 1 to 4, which enable the gain of the repeaters to be adjusted without producing singing. In another arrangement, shown for ordinary and phantom working in Figs. 9 and 10 respectively, pairs of known two-way single-valve repeaters, Fig. 7, are used, the Specification stating that these repeaters function as negative impedances for inputs in one direction. The principle underlying the use of pairs of negative impedances in the first arrangements will be explained with reference to Fig. 1 which shows line sections E, W of characteristic impedance Zo coupled by a hybrid coil to impedances c, d. When impedances c, d are equal and of value Zo, the transmission loss in either direction is infinite but the coil presents an impedance Zo to each line section. If the values of c, d are varied reciprocally by multiplying one and dividing the other by a real number N, the transmission loss becomes finite and amounts in decibels to 20 log10 1+N/1-N, the impedance presented by the coil to the line sections being still Zo. If N is made negative, the loss becomes a gain and the hybrid coil and associated impedances, which are now negative, operates as a two-way amplifier. Two types of negative impedances are described, both involving the use of a back coupled thermionic valve and the combination, of one of each type in a pair facilitates, as will be seen later, the reciprocal variation of the impedances without producing singing. In the shunt type, Fig. 3<a>, the input and output circuits are connected in parallel across the external impedance represented by Re. The Specification contains a mathematical investigation of the circuit and shows that the total impedance R of the amplifier A is given by the expression RiRo/Ri+Ro+M where Ri, Ro are the input and output impedances of the amplifier and M is the mutual impedance representing the ratio of the output voltage to the input current. If M is made negative, for example by reversal of the output connections, R will become negative if - M >- Ri + Ro and the Specification shows that the amplifier will sing if Re >- - R. The valve circuit shown in Fig. 3 is a practical embodiment of this shunt type, the negative impedance being presented by winding 1. In the series type, Fig. 4<a>, the input and output circuits of the amplifier A are connected in series, and the Specification shows that R = Ri + Ro + M, R becoming negative if - M >- Ri + Ro, and the amplifier sings if Re < - R. Fig. 4 shows an embodiment of this type in which the negative impedance is presented by windings 1. If, therefore, these two types of negative impedances are used for the impedances c, d, Fig. 1, their reciprocal variation may be effected to adjust the gain without causing the repeater to sing. N may be varied either by actual adjustment of the impedances or, preferably, by alteration of the ratios of the transformers a, b, Fig. 1, by means of taps. The application of such a repeater to phantom working is shown in Fig. 6. The repeater for side circuit L1 consists of negative impedances A1, All, of the shunt and series types respectively, connected up by tapped transformers b, a, similar impedances A2, A<1>2 being used for the side circuit L2. The repeater for the phantom circuit comprises a shunt negative impedance A3 connected to a tapped transformer B connected to centre taps of transformers b, b<1>, and a series impedance A<1>3 coupled to the side circuits by tapped transformer f. In order that the negative impedances may vary with frequency according to the same law as the line impedances, networks N - - N5 are inserted in their circuits. The repeater shown in Fig. 9, employing two two-way single-valve repeater circuits, will be explained by consideration of the known two-way circuit shown in Fig. 7. The amplifier A is coupled to the line by a hybrid coil H, and a switch S is provided for reversing the connections of one of the circuits of the amplifier. In one position of the switch, and for transmission east, the output current of the amplifier in line LW is in the same direction as the input current in LW, and this arrangement will be referred to as the " direct " connection. If the switch S is thrown over, the output current in LW flows in the opposite direction to the input. This will be referred to as the " reverse " connection, and it will be seen that a " direct " connection in respect of LW is a " reverse '' connection in respect of LE for transmission west. In Fig, 9, two repeater circuits X, Y of the type shown in Fig. 7, are respectively in " direct " and " reverse " connection with respect to LW and are coupled to the hybrid coil H1 by further coils H2, H3. Condensers C2, C3 are inserted between Y and the coil HI in order that there may be an uninterrupted and unshunted path from LW to LE for direct current and currents of such low frequency that the repeater cannot amplify, so that a by-path for such currents around the repeater is not required. Balancing networks N1, N2 are provided and condenser C1 and resistance R1 are added to balance the condensers C2, C3 and resistance of the coil H1. The outputs of the two amplifiers oppose each other in the line section incoming from the transmitter and assist each other in the outgoing line section. ' It has been assumed that no phase shift occurs in the circuits X, Y. If the phase shifts in the two circuits are equal, the operation of the repeater is unaffected, but if they are unequal, condensers C2, C3 are chosen so as to give equal shifts. This type of repeater may be applied to phantom working, as shown in Fig. 10, so that the metallic continuity of the lines is preserved. Each side circuit L, L<1> is provided with a twin repeater As1, A<1>s1 and As2, A<1>s2 similar to the repeater of Fig. 9 except that A<1>s1 and A's2 are connected up by transformers T2, T<1>2. One of the twin repeaters A of the phantom circuit is connected to centre tappings of transformers T2, T<1>2 and the other is conected by transformer T6 to the side circuits. The Specification as open to inspection under Sect. 91 (3) (a) comprises also the negative impedances shown in Figs. 11 (Cancelled) and 12 (Cancelled) adapted to be used in series or shunt with a line. The arrangement shown, Fig. 11 (Cancelled), comprises two valves 111, 112 coupled by resistance 1110 and having their grids biassed by batteries 1112, 1113. The plate voltage for the valves is fed through resistances 1110 and R1, a branch from plate 117 being taken through battery 1118 to input terminal 1117 of the booster. The battery 1118 is so poled and of such a voltage that the normal potentials of the input terminals 1116, 1117 are the same. The input impedance is represented by Z and the external impedance by Z<1>. Any voltage applied across Z is amplified by the valves, and owing to the high resistance of R1, a large portion of the outpub current is fed over 1118 to terminal 1117 and thence back to impedance Z. As the feed back is in phase with the input current, the circuit to the right of A-A acts as a negative impedance. The negative impedance may be made to match any external impedance Z<1> consisting of a resistance, inductance or capacity, or combinations thereof, by building up the input impedance Z of corresponding positive elements according to formulae given in the Specification. In the single valve modification shown in Fig. 12 (Cancelled), the output circuit of valve 1223 includes a network NW and resistance 1235 from which current is fed back to the input circuit by a transformer 1237 which is connected so as to effect the phase change produced by the second valve in Fig. 11 (Cancelled). The value of the negative impedance is adjusted by potentiometer 1227 which is of high resistance so as to impose a negligible load on the transformer. The circuit may be used as a series booster by coupling it to the line by transformer 1232. The resistance 1235 is of sufficient value to give a feed back which neutralizes the plate impedance of the valve, and network NW is built up of elements so as to balance the line impedance as seen through transformer 1232, the network being preferably connected in circuit by a transformer. The negative impedance shown in Fig. 11 (Cancelled) may similarly be used as a series booster, and either may be used as a shunt booster by connecting its terminals to a transformer shunted across the line, the network NW being in this case made to balance the impedances of the line sections in parallel. This subject-matter does not appear in the Specification as accepted.