. The Röntgen rays in medical work . l falls offat 40° C. It is not necessary to increase the surface of thealuminium rod in proportion to the increased volume of surface of 10 square inches of aluminium is sufficient for fairlylarge currents, as the valve effect is best when the current densityat the surface of the aluminium is moderately high. The choice among these methods of employing alternating mainswill depend upon the worker. For least expense, the use of thealuminium valve for charging accumulators, and the working ofthe coil from these with the Mackenzie Davidson interr
. The Röntgen rays in medical work . l falls offat 40° C. It is not necessary to increase the surface of thealuminium rod in proportion to the increased volume of surface of 10 square inches of aluminium is sufficient for fairlylarge currents, as the valve effect is best when the current densityat the surface of the aluminium is moderately high. The choice among these methods of employing alternating mainswill depend upon the worker. For least expense, the use of thealuminium valve for charging accumulators, and the working ofthe coil from these with the Mackenzie Davidson interrupter, ispreferable. All the methods of utilizing alternating mains which have beenenumerated are methods of adapting that type of current to directcurrent types of apparatus, and are of the nature of makeshift THE ELECTRICAL APPARATUS 79 contrivances. The natural way of utilizing alternating circuitswould seem to lie in the direction of step-up transformers capableof providing the necessary voltage for an x-ray tube. This tempting. Fig. 47.—Gaiffes Transformer Apparatus for X-Rays (MedicalSupply Association). short-cut has attracted investigators since the time of the firstdiscovery of x-rays, but the practical difficulties of the method haveprevented the realization of the idea until quite recently. Villard, in1899, had already advanced the matter very notably by his device 80 THE RONTGEN RA YS IN MEDICAL WORK of a valve-tube, and by suggesting the employment of condensersof small capacity to limit the output of the transformer, and so toprotect the tubes. Other obstacles of no small magnitude arose Alternating jrvaon
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