. Elementary and dental radiography . il U tt. J— tt ya n X-/?Ay COIL Fig. 19. Diagrammatic illustration showing the action of an electric switch. The singlearrows mark the course of what we shall call (not knowing a better name) the originalcircuit. The double arrows mark an extended circuit. The drawing shows the switch—one of the double knife variety—closed. With the switch open the current couldnot ]:ass through the extended circuit. X-RAV MACHINES 23 When the X-rays are used for their therapeutic value, long ex-posures are made; so long that unrlue heating of the electrolytic inter-rupter

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. Elementary and dental radiography . il U tt. J— tt ya n X-/?Ay COIL Fig. 19. Diagrammatic illustration showing the action of an electric switch. The singlearrows mark the course of what we shall call (not knowing a better name) the originalcircuit. The double arrows mark an extended circuit. The drawing shows the switch—one of the double knife variety—closed. With the switch open the current couldnot ]:ass through the extended circuit. X-RAV MACHINES 23 When the X-rays are used for their therapeutic value, long ex-posures are made; so long that unrlue heating of the electrolytic inter-rupter would be sure to occur. Hence, for this work the mercury turbineinterrupter (Fig. 21) is best. In principle the mercury turbine is amechanical interrupter, depending on no chemical change for its action,being operated by means of an electric motor. We shall not consider it

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