759,681. Automatic exchange systems. GENERAL ELECTRIC CO., Ltd. Oct. 6, 1950 [Oct. 26, 1949], No. 27505/49. Class 40 (4). [Also in Group XL (c)] Apparatus for identifying a free channel out of several channels comprises means for allocating voltages having different predetermined characteristics to the respective channels, and means for applying said voltages to an " interrogating " circuit which comprises means for preventing voltages associated with busy channels from reaching its output. The voltages may be characterized by a pulse position in a cycle of such pulse positions or by frequency amplitude or waveform (e.g. by coded pulses). Other features are claimed in Specification 759,683. In the embodiment shown in Fig. 5, each of 100 outlets is identified by being associated with a time position in a distributer cycle. The pulse generator 14 is arranged to produce " D "pulses D0 . . . D9 over leads 75 and " U " pulses U0 . . . U9 over leads 74, each D pulse having a duration equal to that of 10 U pulses so that the distributer cycle comprises 100 U pulses and 10 D pulses. The U pulses are also distributed over 100 leads such as P08 . . . P72 each associated with an outlet such as T8 . . . T72. These pulses are applied to a combining circuit 78 whose output, which contains every U pulse, is passed via the interrogating circuit 16 to the lock circuits 34 of all the circuits 10 associated with operator's jacks 11. When a plug is inserted in the jack 11, relay 12 operates to enable the lock circuit 34 which then passes the next U pulse it receives but no others to each of the two sets of 10 switches 76, 77. Each of the ten switches 76 is fed with a different D pulse and each of the switches 77 with a different U pulse. In the set of switches 76 one of the switches receives a D pulse at the same time as it receives a U pulse from the lock circuit and is consequently triggered to cause it to pass all the succeeding D pulses it receives. Likewise one of the switches 77 is triggered to pass all the succeeding U pulses it receives. These output pulses pass to rectifiers 79 and 81 which, when the pulses coincide cause valve 36 to pass a pulse to the common lead 37. Pulses from lead 37 are passed via amplifier 38 and limiter 47 to the interrogating circuit 16 in which they prevent all the U pulses coinciding therewith from passing to the lock circuits for seizure by other jack circuits associated therewith. The pulses from valves 36 are fed to gate circuits such as G8 . . . G72 and gated therefrom to the appropriate outlets T8 . . . T72 by pulses over leads P08 . . . P72 from the pulse generator 14. Dialling impulses over jack 11 are applied to valve 36 to modulate the pulses over lead 37 and these pulses are demodulated by filters F8 . . . F72 before passing to the appropriate outlets. The electronic method of connection as here described is only a temporary one pending the operation of an electromagnetic switch. Thus the firing of the appropriate switches in the sets 76, 77 may mark vertical and horizontal marking banks 88, 89 of the electromagnetic switch 83 whose wiper 85 is then set in accordance with these markings. The gates G8 . . . G72 may operate relays R8 . . . R72 which pass pulses from leads P08 . . . P72 through the combining circuit 82 to the interrogating circuit 16 wherein they prevent the feeding of corresponding pulses to the lock circuit 34. The electronic arrangement may then be switched out by means not shown. In an embodiment, Fig. 7, in which a jack 11 has access to 10 groups of 100 junctions each, the pulse source provides an additional series of " C " pulses, ten of which occur during the time occupied by one U pulse. Each C pulse is associated with one of the ten groups of junctions, the D and U pulses identifying each of the 100 junction lines. When the jack 11 is plugged up and a digit indicating the required group is dialled the switch 92 is set in the usual manner. Each bank terminal 94 is connected to a different C pulse. D and U pulses are connected as before to the two sets of 10 switches 76, 77. All the C pulses are fed from the pulse generator 14 via the interrogating circuit 16 to the gate 95 which is also fed by C pulses identifying the required junction group so that only C pulses of that group are passed to the lock circuit 34 and thence to the switches 76, 77. Two of these switches then trip to pass D and U pulses to rectifiers 79, 81 and when these coincide with a C pulse received over switch 92 valve 36 passes a pulse identifying the junction to be used. This pulse is applied to the interrogating circuit to prevent further C pulses coincident with it from passing to the gate 95. The pulses from valve 36 are applied to ten gates 96 which are opened by the respective C pulses whereby the former pulses are admitted to 100 equipments 97<1> of which there is one for each j unction of the group. The diode 100 of each equipment 97<1> is fed with one of the C pulses so that in one of these equipments the pulse from valve 96 is admitted to charge condenser 101 which is caused to retain its charge for an interval nearly equal to that of the period of the multiplex cycle, after which it is discharged by the application to diode 103 of a negative C pulse immediately prior to the next C pulse applied to diode 101. The thus-lengthened pulses are amplified at 104, demodulated by filter such as F582 and applied to the outlet T582. Transmission of dialling impulses may take place by modulating the pulses in valve 36 as previously described, 100-outlet mechanical switches 83 connected to the bank 105 of switch 92 subsequently replacing the electronic connection as previously described. If required, by grading the pulses applied to the incoming unit 10 (e.g. by inserting a gate between the point 35 and the unit) it may be arranged that each unit has access only to a fraction of the junctions of each group. The pulses from valves 96 may be amplified and broadened before being applied to the circuits 97<1> Thus pulses C1, C2, C3 may be lengthened to the instant when the next C0 pulse occurs and pulses C4 . . . C9, C0 may be delayed until the next C1 pulse occurs and then lengthened until the next C0 pulse occurs. The lengthened pulses would then be gated through the units 97<1> by appropriate U pulses. Where access to a large number of outlets is required the recurrence frequency of the pulses may be so low that they cannot be satisfactorily modulated by speech signals. In the arrangement shown in Fig. 17, low-frequency D and U pulses are used for identifying the junction to be seized and high-frequency D and U pulses are used for carrying the modulation. Thus U pulses are applied over the interrogating circuit 16 and the lock-circuit 34 to the two groups of switches 76, 77 which are supplied over cables 24<1>, 24 with low-frequency D and U pulses. When one switch in each group has been tripped they pass corresponding high-frequency D and U pulses from cables 2411, 24<1> to valve 36. A channel identifying pulse is thereby applied over switch 175 to the equipment associated with the wanted group of junctions. These pulses are gated to the appropriate outlet by high-frequency D and U pulses applied to terminals 193, 194. When the demodulator 186 operates it opens a pulse gate 195 to admit low-frequency D and U pulses whereupon it passes a U pulse, coincident with that identifying the junction, to the interrogating circuit 16 to prevent the passing of that pulse to the lock-circuit 34. The return speech modulates high-frequency pulses derived from the coincidence of high-frequency D and U pulses and fed to the incoming circuit 10 where it is selected by the gate 206, which is opened in synchronism with gate 36; and the output is demodulated by filter 207. An arrangement is also described in which a binary scale is used for identification, Figs. 24, 25 (not shown), in which the two sets of 10 switches 76, 77 are replaced by a number of sets of 2 switches fed by pulses of length 1, 2, 4, 8 . . . times the length of the pulse being identified. In a further embodiment, Fig. 26, the various outlets are distinguished by being associated with a characteristic carrier frequency. The generator 14 applies the characteristic oscillation cyclically to the lock circuit 34 over the continuously rotating switch 295. The first frequency to be applied after relay 12 operates, passes through the lock-circuit 34 and operates a corresponding one of the switches 25 . . . 29 which thereafter passes that frequency (applied over one of the leads 30) to the valve 36. Here it is modulated by the incoming signals and applied via the corresponding one of the bandpass filters 301 . . . 305 to one of the junctions T1 . . . T5. The frequency is also applied via one of the band-pass filters 306 . . . 310 to close the corresponding one of the gates 285 . . . 289 to prevent that frequency being subsequently applied to the lock circuit by the switch 295.