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The tungsten cathode contains connecting wires, a filament, and a focusing cup. The filament needs to be heated for thermionic emission to take place. The filament circuit is supplied by a voltage step-down transformer through which 220 V is stepped down to 10 V with a resulting filament current of 3 to 5 A 1.
A small change in filament current can cause a large change in tube current, and the filament current is controlled by the filament voltage due to Ohms law:
Volts = current (amperes) x resistance (ohms)
A change in filament voltage of about 5% will cause a 20 to 30% change in tube current 1.
Besides, by altering the voltage of the focusing cup, the tube current can also be controlled. When the voltage applied on the focusing cup is increased, emitted electrons are pushed closer together, which will eventually pinch off the tube current when the voltage is large enough 1.
Filament current should not be confused with tube current which the latter determines the flow of electrons from the cathode to the anode. The tube voltage (kVp) and tube current (mA) is supplied by a voltage step-up transformer, where the voltage of the tube is stepped up from 220 V to 75 kV with a resulting tube current of 100 to 200 mA 1.