243,088. Western Electric Co., Ltd., (Western Electric Co., Inc.). Aug. 27, 1924. Thermionic generators and amplifiers.- Transmitting equip - ment of the type described in Specification 217,552. comprising a number of high-powered amplifying valves in parallel, is characterized by the provision of means for preventing " singing " between assorted banks of amplifiers. An aerial coupling arrangement claimed in Specification 242,928 permits transmission characteristic of the antenna to be broadened to cover the side band frequencies. An associated equipment for measuring the effective modulation of the radiated energy is also provided. General circuit outlay. A typical circuit arrangement is shown in Fig. 1. Speechfrequency currents from a microphone 12 are impressed upon a balanced modulator 19 of the type described in Specification 130,219, and fed with a carrier wave from a high-frequency source 16. One of the resultant side bands is separated out by a filter 20 of the kind described in Specification 142,115, and fed to a second modulator 22 supplied by a separate carrier frequency from the source 23. One of the resultant side bands is again separated by a filter 24 and applied to an intermediate-power amplifier 25. From here the energy is pased to a high-power amplifier comprising two units 73, 74 each consisting of a bank of thermionic tubes arranged in parallel as described in Specification 217,552. Singing-preventing means. The input voltage across the resistance 36 is applied to the amplifier banks in parallel through a reactance device 37, 38 designed to prevent " singing " or reaction effects between the two units. The reaction device consists of an inductance 37 connected in parallel with a resistance 38. The input energy is applied to the middle or to an intermediate point of this arrangement, the two halves of the induction coil being closely coupled so as to offer no impedance, the outer terminals being connected respectively to the input circuits of the units 73, 74. Whilst the device offers low impedance to the passage of currents of the frequency to be amplified, the coil 37 possesses a high impedance to undesired high-frequency impulses arising from capacitative back-coupling between the units. Such frequencies are accordingly forced through the resistance path 38 of the device and are there largely dissipated. The reactance device may be applied to the output from the amplifier banks. Fig. 4 represents the application of similar reaction devices for preventing " singing " effects between an odd number, namely three, of amplifying units 120, 121, and 122. Here the two reactive devices 123, 124 each comprise inductances 125, 126, 127 connected in a closed loop with resistances 138, 139, 141, taken to a common junction, whilst switches 145, 146, 147 are arranged to short circuit the inductances if required. Fig. 5 represents another reactive anti-singing arrangement comprising reactances and resistances combined with switching means, as shown, for connecting any number of amplifying units 152 in parallel and also for switching out one or any number of units without disturbing the operation of the others. Coupling oscillating and radiating circuits.- The output from the power amplifiers 73, 74, Fig. 1, is passed to a coupling circuit 78 comprising a variometer 90 in series with a coil 91 coupled to a directive antenna of the type in which a number of down leads each contain inductances. The coupling coil 91 is shunted by a branch comprising a resistance 97 in series with a condenser 98 for by-passing the higher harmonics away from the aerial circuit. The result is stated to broaden the effective transmission characteristic of the antenna. Measuring and testing apparatus.-In order to indicate the effective modulation, and so maintain a proper factor of safetv a series of three valves 103, 104, 105 are coupled at 101, 102 to the aerial circuit. The respective grid potentials are so graded that as the percentage modulation increases the ammeter A in the plate of the tube 102 is first operated, then the ammeter A of the tube 104, and finally an excessive modulation is registered by the ammeter of the last tube 105.