. The principles and practice of roentgenological technique . nput donot exist to the same degree, so that whenspeed, detail and sharpness of image are re-quired the gas tube is preferable. 5. Timing of Exposure An accurate record of tissue density in tonevalue and a sharply defined true shadow oftissue form would be relatively simple with aparallel ray, but radiography has to do witha divergent ray. The homogeneity of abundle of divergent rays, falling upon a givensurface may be increased, first, bv decreasingthe focal spot, secondly, by increasing the tar-get recording surface distance. The
. The principles and practice of roentgenological technique . nput donot exist to the same degree, so that whenspeed, detail and sharpness of image are re-quired the gas tube is preferable. 5. Timing of Exposure An accurate record of tissue density in tonevalue and a sharply defined true shadow oftissue form would be relatively simple with aparallel ray, but radiography has to do witha divergent ray. The homogeneity of abundle of divergent rays, falling upon a givensurface may be increased, first, bv decreasingthe focal spot, secondly, by increasing the tar-get recording surface distance. The result ofthe application of one or both of these con-ditions is to increase the sharpness of the re-sulting shadow and to minimize its still further increase the definition and prevent the loss of detail, which results fromscattered and secondary radiations, it becomesnecessary to utilize such adjuvant means asdiaphragms, cones, cylinders and the Buckydiaphragm. The result of the utilization ofall these factors is to increase the Fig. ^85.—Automatic time switch and breaker(Wappler) The exposure may be timed by means ofmetronome or an automatic timing device. Where instantaneous exposures are madethe automatic time switch is to be usually consists of a timing elementand a mercury circuit breaker metronome does not aid in the control ofexposures of less than one second. The serial timer and breaker (\\appler)consists of tv\o main parts, a timing deviceand a device for the interruption of the pri-mary current of the transformer. The timing device contains an air bellows,which has an escape valve that can be regu-lated to close electric contacts for the variouslengths of time. Closing the circuit, actuatesan electro-magnet, the armature of whichcarries a pair of copper rods, which make con-tact with the mercury contained in two sep-arate iron pots. In each pot is placed 34 lb. of pure mercurycovered with oil reaching
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectradiogr, bookyear1920
