606,496. Control systems for resistance welding etc.; automatic control. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 16, 1942, No. 17866. Convention date, Dec. 18, 1941. [Class 38 (iv)] Thermionic control.-Relates to the control of valve apparatus interconnecting an alternating current circuit and a load circuit, such as valves reversely connected in parallel to supply a predetermined number of half-waves to a welding circuit. A controlling valve has excitation means for impressing on its control member a periodic voltage variable in phase with respect to the supply voltage, the excitation means comprising a phase-shifting circuit controlled in phase by the potential on the grid of a discharge device and this potential being varied in accordance with load circuit current to maintain that current substantially constant. To prevent excessive current on establishment of the load circuit through the valve apparatus, means are provided to limit the phase displacement, prior to such establishment, these means being rendered inoperative in response to load current; as described, this inoperativeness occurs gradually. The Figure shows ignitrons 4, 5 interconnecting an A.C. supply 2 with a welding load 1 and triggered by grid-controlled discharge devices 12, 13. The grids 17 of these valves receive voltages compounded of a phaseregulated voltage from a winding 39 of a peaking transformer, a hold-off voltage from a winding 26 in phase with the supply voltage and a voltage from a winding 35 energized in opposed phase when valves 86, 87 are rendered conducting by a timing circuit 42 and including a timing condenser 56. The phase of winding 39 is determined by a reactor 117 of which a coupled winding 118 may be variably loaded by rectifying valves 121, 122, the voltage on grids 123 of which is made to depend on the load current. Thus, the rectified output of a current transformer 125 gives a voltage on a resistance 127 which is applied to the grid of a valve 137 to draw a proportional current through a resistance 138 from a constant potential source 132 and so vary the voltage on grids 123. When no current is passing in transformer 125, a valve 145 determines this potential, its grid 146 receiving a voltage from source 132 causing it to load resistance 138, but as soon as current is established in the ignitions 4, 5 a relay 150 connected across a shunt 151 disconnects grid 146 from source 132, whereupon its voltage dies away at a rate determined by the condenser 152 and resistances 148 in its grid circuit, permitting valve 137 to take over. The welding system may be put into operation by closing the switch 6, whereby the time delay relay 106 is caused to establish a connection between a D.C. source, which may be derived from a rectifying arrangement 40 and a condenser 55, and when this condenser has charged, the anode circuits of the trigger valves 12, 13 are closed by relay 109. To commence welding, switch 81 is closed to actuate relay 83 and thereby open the discharge circuit of timing condenser 56 and complete its charging circuit from the D.C. source through the control valve 58. To fix accurately the relation between the commencement of this charge and the A.C. cycle, a suitably phased peaking wave is applied to the grid 59 by a transformer 60. The voltage of the condenser 56 is applied in opposition to an adjustable voltage derived through a potentiometer 65 ... 70 from the D.C. source to the grid of valve 86 so that the period during which the trigger valves 12, 13 are operative may be determined. Valve 87 trails valve 86 and these valves energize transformer primary 35. Transformer primary 91 is energized in suitable phase in parallel with primary 35 to eliminate fluttering of valves 86, 87 in known manner.