. A text-book of radiology . le of the prevents the tube becoming discoloured duringexhaustion, and also saves the anti-cathode from damageto its surface. The anti-cathode is usually of nickelwith a thin coating of platinum, and while heavier thanthat of the tube just described, it still belongs to the light anode variety. It is provided with a regulating device which will berecognised as the mica disc variety from the descriptionalready given. It also possesses the small bulb forraising the vacuum. The regenerating device is automatic—and by settingthe hinged wire at any given dista
. A text-book of radiology . le of the prevents the tube becoming discoloured duringexhaustion, and also saves the anti-cathode from damageto its surface. The anti-cathode is usually of nickelwith a thin coating of platinum, and while heavier thanthat of the tube just described, it still belongs to the light anode variety. It is provided with a regulating device which will berecognised as the mica disc variety from the descriptionalready given. It also possesses the small bulb forraising the vacuum. The regenerating device is automatic—and by settingthe hinged wire at any given distance from the cathodeend the tube will regulate itself to the vacuum corres- LIGHT ANODE X-RAY TUBES. 51 ponding to that distance, for as the resistance of the pathbetween the proper electrodes increases, current passesaround bj^ the mica discs down the wire and jumps acrossto the negative end. This sets free a minute quantity ofgas from the mica and the vacuum is lowered, allowingthe current to pass in the usual way Fig. 19. Heavy Anode Tube. If a new tube of this type be worked for some timewith a moderately strong current it will very soon befound that the vacuum is getting too low. It can besaved if the current is reduced at once, or cut offentirely so as to let the tube cool. If this is not done intime considerable shortening of the useful life of thetube will take place. On the other hand, if at first it is only used forradiography of the thinner parts as hands and toes, or ifin treatment, care is taken to pass but a very smallcurrent, the tube will not only be found to remainsteady or even rise in vacuum, but concurrently itgradually develops an increasing degree of stability. 52 TEXT-BOOK OF RADIOLOGY. A stage may eventually arise when it will not onlyeasily stand double or treble the amount of current, butwill do so for more or less prolonged periods of time. A tube in this condition is said to have become seasoned. This seasoning of a tube is an art, but awel
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