. The principles and practice of roentgenological technique . n of the target with metallic deposit onthe glass absorbs a certain quota of the , reversing the polarity, which causesmarked sputtering should be avoided. As the tube hardens, the color changes toa light gray green, the division zone becomesindistinct and the hemispheres lack contrast. the accessory anode. The cathode shows itsbrightly glowing concave surface. The stemand the glass extension in which it is set aredark. In medium and low vacuum tubes afan-shaped, illuminated area coming from thecathode is visible. The cath
. The principles and practice of roentgenological technique . n of the target with metallic deposit onthe glass absorbs a certain quota of the , reversing the polarity, which causesmarked sputtering should be avoided. As the tube hardens, the color changes toa light gray green, the division zone becomesindistinct and the hemispheres lack contrast. the accessory anode. The cathode shows itsbrightly glowing concave surface. The stemand the glass extension in which it is set aredark. In medium and low vacuum tubes afan-shaped, illuminated area coming from thecathode is visible. The cathode stream can be recognizedby the eye only in very soft tubes, but is ap-parent in the photograph in all except veryhard tubes. The luminosity is due to theslowly moving electrons which impinge uponthe gas molecules and cause them to fluor-esce. This phenomenon may be seen in low •84 PHOTOGRAPHY OF FOCAL POINT OF TARGET vacuum tubes as a stream of electrons extend-ing from the target to the glass wall. Thebrilliantly lit area on the target represents the. Fig. 107.—Fine focus medium vacuum tube, run atnormal pressure the target completeb utilizesthe cathodal discharge. point of impact of the cathode stream. Thisfocal spot grows larger and brighter as the-Stream broadens. In front of it is a glowingarea—the reflection from the focal point. A reversed image of the cathode cup maybe seen immediately above the cathode. Thisconsists of an illuminated ring, correspondingto the base of the cone of the cathode stream,and a bright center corresponding to the focusof the cathode ray bundle. Photographic studies of the activated tubepermit the determination of the sharpness ofthe focus, its size and stability. A fluorescentscreen may be used instead of a plate for rapidtests. The study of the focal spot has an im-portant bearing on the sharpness of the shadowcast and upon the amount of electrical energ}^the tube is capable of consuming for theproduction of x-rays without disintegra
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectradiogr, bookyear1920
