636,357. Automatic current control systems; rectifying. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Aug. 8, 1947, No. 21835. Convention date, Oct. 9, 1941. [Classes 38 (ii) and 38 (iv)] The space current of an X-ray tube fed with rectified H.T. is maintained constant by variation of the cathode heater current in response to changes in space current. A transformer 5 having double secondary windings 9, 10 connected to pairs of rectifiers 13, 14 ; 17, 18 provides a full-wave rectified, unsmoothed, high-tension supply for the X-ray tube 12..The circuit includes a variable series resistor 20, the voltage drop across which is proportional to the space current of the tube. A gaseous discharge device 32 is connected through resistors 33, 34, in parallel with resistor 20. If the space current increases, the voltage peaks, due to the unsmoothed supply, which appear across resistor 20 cause device 32 to conduct periodically and place a negative bias voltage on the grid of valve 36, reducing the current in the control winding of a saturable reactor 30 connected in the anode circuit of valve 36. The D.C. control current is derived from a transformer 40 and rectifier 48. The saturable reactor being in series with the primary of transformer 25 controls the heater current of the tube cathode to restore the space current to its predetermined value. In a modification, Fig. 2 (not shown), the tube H.T. supply is derived from a transformer having a split secondary supplying a bridge rectifier.