Patent GB-163487-A (GB)
Title: Improvements in and relating to the detection of high frequency electric signalling currents
IPC: H01J21/18
Filed: 1920-02-20
Abstract: 163,487. British Thomson - Houston Co., Ltd., (General Electric Co.). Feb. 20, 1920. Vacuum tubes.-A thermionic device for receiving high-frequency signals is designed to possess an internal resistance which substantially follows Ohms law in that the output current is directly proportional to the impressed voltage. The necessary asymmetry for securing a rectifying effect is secured by periodically varying the effective value of this resistance by local means which are independent of the current in the receiving-circuit. The thermionic device 1 consists of an evacuated vessel containing two heated electrodes 2, 3 connected to the receiving circuit 9, 10, as shown in Fig. 1. By means of a coil 6 surrounding the vessel 1 and energized from a local source 7, an alternating magnetic field, parallel to the electrodes 2, 3, may be superposed upon the electron flow within the vessel, and will thereby, in effect, vary the net resistance of the device to the impressed voltages from the circuit 9, 10. The curve K, Fig. 2, may be taken to represent the normal electron flow (characteristic of any two-electrode thermionic device) from electrode 2 to electrode 3, Fig. 1, as the potential of 3 is increased by surges in the circuit 9, 10; simultaneously the curve L will represent the electron flow from electrode 3 to electrode 2. The combination of these two curves, and the resultant current through the device may therefore be represented by the straight line M. The effect of the superposed magnetic field from the coil 6 upon the conductivity of the device is shown in Fig. 3. When there is no magnetic field, the electron flow is undisturbed and the conductivity is at a maximum OA. A strong magnetic field in either direction (represented by + or -) will divert the electrons along the path of the field, parallel to the electrodes, and will practically shut off all current, so that the conductivity of the device sinks to 'a minimum C. The effect of superposing a magnetic field of a frequency slightly different from half the frequency of the signalling current is shown in Fig. 7, where H represents the undamped signal variations. G the impressed magnetic field, I the resultant high-frequency variations of current passed through the device, and J the net low-frequency current actuating the telephones 11. Fig. 8 shows a modified form of detector comprising a heated filament 13 surrounded by an anode 14. The normal characteristic curve of such a device would be similar to K, Fig. 2; but by adjusting the phase of the superimposed magnetic field so as to reduce the current to zero when the impressed signal voltage falls to zero, the effective resistance will approximate to the straight line M. As shown, current will only flow through the device in one direction, but this suffices to operate the telephones. By using two such devices, both halves of the signal currents may be utilized. If the frequency of the magnetic field is first made half that of the input energy; and the two effects are then synchronized in phase, the output current will be simply rectified, and the resultant direct-current impulses maybe recorded upon a photographic receiver.