to a series circuit consisting of a normally open contact of the portion switch s 1 a coil of an auxiliary relay y 1 of the portion switch s 1 and normally closed contacts of the portion switches s 2 s 3 to a series circuit consisting of a normally open contact of the portion switch s 2 a coil of an auxiliary relay y 2 of the portion switch s 2 and normally closed contacts of the portion switches s 3 s 1 and to a series circuit consisting of a normally open contact of the portion switch s 3 a coil of an auxiliary relay y 3 of the portion switch s 3 and normally closed contacts of the portion switches s 1 s 2 whereby these three series circuits are parallel to each other the a c power source 3 is also connected to an autotransformer t 1 three output taps of this autotransformer t 1 are connected to one end of a high voltage transformer t 2 on its primary side through the respective contacts of the relays y 3 y 2 y 1 and also through a ct contact the other end of the high voltage transformer t 2 on its primary side is connected to the other output tap of the autotransformer t 1 the secondary side of the high voltage transformer t 2 is connected via a rectification circuit 5 to the cathode and anode target 7 of the x ray tube 6 and impresses a high voltage upon the same when each portion switch s 1 s 2 s 3 is turned on the tube voltage is primarily and.