Changes in X-Ray Tubes by Use.

I, By repeated use an X-ray tube becomes progressively harder. This is due to (a) inverse currents, and (6) escape, or occlusion of gaseous particles from the interior of the tube.

(a) The formation and prevention of inverse currents are’discussed later, when induction-coils fall to be con- sidered. Meanwhile, we note that inverse currents may originate in the secondary winding of the induction-coil, and pass through the tube as discharge in a reversed direction. In their passage these currents tear from the platinum antikathode fine particles, which absorb or occlude electrons from the rarefied contents of the tube and thus increase the degree of vacuum. Presence of such inverse currents may be noted in the action of the tube, since they produce a flickering of the fluorescence, specially noticeable in the hemisphere normally dark.

Due to this action, the fluorescent hemisphere of a tube, subjected to such adverse influence, becomes blackened by deposit of finely disintegrated metallic particles, in contrast with the violet tint due to chemical change acquired by a tube guarded from inverse currents. To obviate disintegration during correct operation of a tube, the kathode is generally made of aluminium, which is found to resist such action more than any other metal tried.

(b) Escape of electrons may occur by piercing the glass of a tube, impulse from within being so much greater than any pressure from without. The degree of vacuum in the tube is thereby directly raised.

II. During each operation there tends to be (a) a pro- gressive softening, or, under exceptional circumstances, there may be (b) a slight hardening.

(a) A progressive softening is noted when a tube is so operated that the antikathode becomes overheated by the continual bombardment of the kathode rays. This heating has the effect of liberating electrons otherwise held bound by the metal, and by the liberation of these into the space of the tube the degree of vacuum is lowered.

If the tube be over-driven, this effect may become so marked as to reduce the equivalent spark-gap to nil. In operating a tube, its condition should be observed periodically by approaching the discharging points of the coil or noting the reading of the milliamperemeter. If softening of the tube be indicated by a marked shortening of the alternative spark-gap, or rise in the reading of the milliamperemeter, care must be observed that the tube does not receive injury, or the patient be exposed to risk of over-effect. In such event it will be well to decrease the amount of current employed or to give the tube time to cool.

In use the antikathode should never be allowed to get hotter than indicated by a cherry-red colour, unless softening of the tube is desired for special photographic effects, as will be described later.

If the kathode rays were focussed to an actual point on the antikathode the metal would readily become fused by the heat. Therefore in practice the target, made of platinum, is placed a little to one side axially of the focal point, and the X rays originate from a small circular area measuring between TV and J inch in diameter. To permit of a nearer approach to the true focus, combined with prolonged use, it has been suggested to make the target of osmium or iridium on account of their greater hardness and infusibility, but the expense and trouble in working of those metals are incommensurate with the advantage to be gained by their use. This point is further discussed later (p. 20).

The focal area on the target is usually indicated by a slight roughening of the metal, the test-running by the maker being sufficient to produce this effect, and it should be looked to in selecting a tube. If larger than £ inch in diameter, there will be lack of definition in shadows cast by the tube.

(b) Slight hardening of the tube may occur, or softening process be delayed, by the presence near the tube of an insulated mass of metal such as in a diaphragm. Such metal acts as a condenser, and, holding electrons bound to the adjoining inner surface of the tube, reduces the number free to occupy the tube space, and thereby raises the degree of vacuum. This effect may be obviated by connecting to earth either the kathodal wire or the metal mass in question.

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