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The toroidal confinement and the ohmic heating of a thermonuclear plasma as used in tokamak experiments can be achieved by strong magnetic fields and by a heavy direct current flowing parallel to the axis of the plasma torus. This current can be induced with the aid of a transformer for which the plasma torus acts as the secondary winding. The high loop voltage required for start-up is obtained by commutation of the primary current to a resistor. This commutation can be achieved by a mechanical switch or by electronic circuits. New electronic commutation circuits are described which enable very heavy direct currents to be switched without contacts. The operation of the new circuits was checked by computer calculations. Experiments have not yet been performed.