451,346. Wireless receiving-apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Assignees of Barton, L. E. ; 228, Burrwood Avenue, Collingswood, New Jersey, U.S.A.) Dec. 3, 1934, Nos. 34748 and 34749. Convention dates, Dec. 1, 1933 and Feb. 28, 1934. [Class 40 (v)] Further rotation of the tuning element of a wireless receiver is automatically prevented, when a desired station has been correctly tuned in, by a device operated automatically in dependence on received signal strength. Fig. 5 shows a superheterodyne receiver in which noise suppression is obtained in part by the action of a pentode 61 connected in series with the second low-frequency amplifier 45 of the receiver, so as to serve as a variable self-biassing resistance for that valve. The noise suppresser control is initiated by a diode-pentode 65, the anode circuit 67 of which is sharply tuned to the intermediate frequency. The control grid is coupled to the end of the signal diode load-resistance 37 by a condenser 81 having high impedance to audio frequencies, the input voltage to the pentode 65 being controlled by a variable condenser in parallel with coupling resistance 83. The amplified intermediate frequency output from 67 is applied to the diode 69, the current of which biases the control grid of the pentode 65 through resistances 101, 99 in accordance with received signal strength. Audio frequency signals are byepassed by condenser 87. The control grid-cathode circuit of the valve 65 comprises resistances 99 and 101, the voltage drops through which are additive. With no signal, the bias due to resistance 99 is small and there is a large voltage drop across the plate resistance 91 ; the gain of valve 65 is thus low due to reduced anode potential. Resistance 91 is connected to a supply potentiometer 93 in shunt with the voltage supply source 95, while its upper and negative end is connected to a tapped potentiometer 111. An intermediate point on 93 is earthed, so that, with no signal, the control grid of valve 61 is rendered highly negative, and the audio frequency amplifier 45 is blocked. When the receiver is exactly tuned to a signal the drop across resistance 91 decreases and the point B on potentiometer 111 becomes less negative. The valve 61 thereby becomes operative and the prohibitive bias on amplifier 45 is removed. A brake control valve 133 has its control grid connected to point D on potentiometer 111 through a grid leak resistance 141 and to point A through condenser 151. With no signal, valve 133 is blocked by prohibitive bias from point D so that no current from this source flows through the brake winding 147 and the brake 135 is inoperative. A small current in the opposite sense is, however, permitted through the circuit from the positive end of the divider 93 through resistance 153, line 145 and winding 147 to a less positive tapping on the divider 93, to remove any residual magnetism in the brake. The blocking bias on valve 133 is removed by the action of the valve 65 by virtue of the rise in potential of point A of potentiometer 111 when the signal is tuned in, as for valve 61. The rise in potential across condenser 151 causes the control grid of valve 133 to assume a less negative potential, the resulting flow of anode current energizing the brake 135 to clamp the dial member 191 when the receiver is exactly in tune to the desired signal. As the potential of point D of 111 is sufficiently negative with respect to earth to give a prohibitive bias on valve 133 even when the signal is tuned in, the brake 135 is automatically released a predetermined time following its actuation. Alternatively, the brake may be released manually by pushing the tuning knob inwards, thus closing a switch in the brake-release circuit ; also, a friction clutch may be provided between the tuning knob and condenser spindle ; also, a relay may be shunted across the brake actuating circuit by means of a switch, to switch in a signal lamp behind the tuning dial when the set is in tune. In order to prevent actuation of the brake by a strong signal adjacent the desired frequency, automatic volume control is applied from the diode 161 of the doublediode-triode 43 to the valves 1, 7, and 21, the sensitivity of the receiver being controlled by variable grid-biassing resistances, unicontrolled with the knob 341. One form of magnetic brake, shown in Figs. 2 and 3, comprises a U-shaped electromagnet 183, the poles of which co-operate with a flat iron bar 187, supported by hinge supports 189. The tuning dial is provided with a disc segment 191, preferably of magnetic material. Remote control of tuning and volume is effected in Fig. 5 by a system of push buttons controlling a reversible motor 337 which drives the tuning shaft 157 and also the volume control spindle 423 by belts and pulleys. Push buttons 341, 343 effect rotation of the motor, forward or reverse, the brake 135 being actuated when the receiver is in tune as previously described. The brake may be released for retuning by pressing button 345. The volume control shaft 423 carries a disc 433 co-operating with a second magnetic brake 421, the winding 441 of which is energized from the potentiometer 93 through push-button 347. When the receiver is in tune and brake 135 is energized, the volume control brake may be released by depressing push button 347, and on depressing button 341 or 343 the volume control may be remotely adjusted; during this operation the tuning is unaffected, as the tuning condenser driving belt merely slips. A further method of remote tuning and volume control comprises a system of bevel gears and a double friction drive between the control motor and tuning and volume control spindles ; the drive to the condenser spindle is through toothed wheels and a metal belt, Fig. 6 (not shown). The tuning dial may be of the type comprising two fingers, geared together like the hands of a clock, Figs. 7 and 8 (not shown). Specification 425,626 is referred to.