. Elementary and dental radiography . rom steel to aluminum and on, but when the currentreverses and starts to flow from aluminum to steel, a chemical changeoccurs in the aluminum, making it a non-conductor and choking off theflow. Thus a current of 60-cycle frequency, after passing through a one-cell rectifier, becomes practically (there is a slight inverse current) a X-RAY MACHINES 25 uni-directional current with 30 interruptions per second. If, after passingthrough the rectifier, as just described, the current is an interrupted one,the questions arise: Why send it through an interrupter? Wh
. Elementary and dental radiography . rom steel to aluminum and on, but when the currentreverses and starts to flow from aluminum to steel, a chemical changeoccurs in the aluminum, making it a non-conductor and choking off theflow. Thus a current of 60-cycle frequency, after passing through a one-cell rectifier, becomes practically (there is a slight inverse current) a X-RAY MACHINES 25 uni-directional current with 30 interruptions per second. If, after passingthrough the rectifier, as just described, the current is an interrupted one,the questions arise: Why send it through an interrupter? Why notdirectly on to the coil? Because the interruptions are not sharp andcomplete enough. The current is pulsating rather than interruupted. By connecting three or four rectifier cells in a certain way (Fig. 28),we are able to obtain practically a uni-directional, constant current. If the supply current is 60-cycle, as is usually the case, the electrolytein the interrupter remains tlie same as for a , i lo-volt circuit namely. j^. Fig. 22. Vibrator or meclianical interrupter. about 20 per cent., but if the supply is 133-cycle, the solution shouldbe stronger—about 30 per cent. From the interrupter the current passes into the rheostat, as perFig. 17. A rheostat (Fig. 29) is an apparatus by the useRbCO$tat of which we are enabled to regulate the quantity of electricity entering an electric machine. The rheostatdoes not have nuich efifect on voltage. Fig. 30 illustrates the rhcustat. A represents coils of wire, oftenGerman silver, offering- great resistance to the flow of electricity. Whenthe arm, I, is on button 1, the current must pass through all the resistive wire on its way to the electric machine, induction coil, motor, orwhat not. This resistive wire chokes back am]~)erage. On button, 2,there is less resistance; o\\ button, 3 still less, until on the last button thecurrent passes direct)}- into the machine. The rheostat illustrated acts 25 ELEMENTARY RADIOGRAPHY a
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