Patent GB-734345-A (GB)
Title: Improvements in or relating to electric signal switching circuits and cold cathode gas-filled discharge tubes therefor
Inventor: BECK ARNOLD HUGH WILLIAM
IPC: H01J17/48
Filed: 1952-04-10
Abstract: 734,345. Multi-gap cold-cathode tubes. STANDARD TELEPHONES & CABLES, Ltd. March 16, 1953. [April 10, 1952], No. 9307/52. Class 39 (1). [Also in Group XL (c)] A cold-cathode gas-filled electric discharge tube comprising two anodes symmetrically disposed with respect to the cathode is used as a switch to open or close a speech circuit connected so as to include the gap between the said anodes. In the embodiment shown the tube comprises rectangular cathode plates 47 and 471 supported on opposite sides of a vertical sheet of mica 44 mounted between mica discs 45 and 46, the cathode plates being spaced from the sheet 44 to avoid trouble due to cathode sputtering. Rod anodes 48, 49, 50 and 51 are mounted with their axes lying in planes parallel to the cathodes. The impedance between the anodes can be reduced by using four anodes in association with each cathode, alternate anodes being connected together. Similarly, six or more anode rods may be used. The anodes may alternatively be in the form of flat strips parallel to one another and to the cathode or at right-angles to the cathode, and cylindrical arrangements are described in which a rod cathode is coaxially surrounded by cylindrical anodes spaced apart along the axis of the cathode, or a cylindrical cathode may surround two axial rod anodes or two rod anodes arranged side-by-side. The gas filling may be helium, argon or neon or mixtures thereof, for example 99 per cent neon and 1 per cent argon at a pressure of 30 mm. of mercury, or helium at a pressure of 70 mm. of mercury. The tube is connected in a circuit (see Group XL (c)) so that the anodes are in parallel for D.C. purposes but are in series for speech currents. The path between the anodes in the gaseous discharge exhibits a low impedance which is substantially resistive and independent of frequency, and noise is not noticeable. A differential characteristic is shown plotting anode current against anode-anode potential difference and indicates that over a limited range of difference in potential between the anodes the total discharge current remains approximately constant. The separation between the anodes of any of the arrangements is not critical, but curves plotting anode-anode impedance as ordinates against the product of anode-cathode separation and gas pressure for various gas fillings show that at low values of the product a fairly flat curve is obtained which rises rapidly at higher product values, and consequently the anode is so located that for abnormal glow discharge between the cathode and anode the anode is situated between the edge of the cathode dark space and the end of that region of the Faraday dark space in which there is substantially no electron space charge sheath surrounding the anode. The statistical delay of firing of the anode-cathode gaps may be eliminated by inserting some radio-active substance within the tube envelope, by subjecting the cathode to light of suitable wavelength, or by an auxiliary discharge. An arrangement is shown in which an auxiliary discharge is shielded by a quartz partition so that the anode-cathode striking voltage is not reduced by ions from the auxiliary discharge. Alternative arrangements comprise auxiliary rod electrodes enclosed in a quartz tube which allows light of the required short wavelength to pass, and a heated tungsten filament which illuminates the cathode. A trigger electrode may be used to fire the cathode-anode gaps.