Gaseous tube selecting switch for use in automatic telephony

670,578. Electric selectors. TELEFONAKTIEBOLAGET L. M. ERICSSON. Dec. 30, 1947 [Jan. 3, 1947], No. 34823/47. Class 40 (iv). [Also in Group XL (a)] In an arc discharge tube, wherein pairs of auxiliary electrodes can be short-circuited by the magnetic deflection of the arc thereto, movement of the arc is arrested at a pair of centring electrodes by the regulation of the magnetic force on the arc through an electron discharge tube. As a preliminary to deflecting the arc on to one of the electrodes 31 corresponding to the selected number, the arc 13 is ignited by closure of a switch in the handset 44, whereby the coil 43 is energized to close relay R1. Contact 45 is thereby closed to strike the main arc, contact 35 is closed, whereby the centring electrodes 32 are connected to a potential through the winding 34 of relay R2, and contact 40 is connected to earth through resistance 46 whereby the arc deflecting field is increased by the continuous increase of the anode current in discharge tube 38 at a rate which is determined by the values of grid condenser 41 and resistance 46. The main arc is held at the position of electrodes 32 by the local discharge current which operates relay R2 to break contact 47 and prevent further increase of the potential of grid 38. The handset is provided with a number of resistances one of which can be selected by a push-button to impose a corresponding potential on the grid 38 whereby the arc will be deflected to the appropriate auxiliary electrode 31. In a modification, deflection of the arc is effected by variation of the arc current within a fixed or varying magnetic field. In another form the valve circuit shown may be replaced by a resistance bridge wherein the magnet coil 14 is connected across one diagonal and the operating voltage across the other; push-buttons in the handsets are adapted to insert in one arm of the bridge an appropriate resistance corresponding to the selected number. Different line resistances are compensated by supplementary resistances preferably inserted in the handsets; these resistances may be disconnected, e.g. by the telephone cradle after the selected circuit has been established. An automatic regulator may fix the line current at a predetermined value for an incoming call. Specification 666,123 is referred to.