Chapter 1 – Part 1

THE X-RAY TUBE

Classification of Tubes.

TYPES and modifications of X-ray tubes are many and varied; but variations are of secondary import, and all conform to the general plan designed by Jackson.

As mentioned in the introductory remarks on the evolution of the X-ray tube, much depends on the degree of vacuum existing in the tube under observation. This vacuum, attained initially by means of a mercury pump, and completed usually by passage of electricity, can be adjusted within certain limits by the maker at the time of manufacture, and a tube may thus be made to suit any specified set of conditions.

A tube in which the exhaustion has not been carried very far is spoken of as of low vacuum, or ‘ soft’; whilst a tube more thoroughly exhausted is by contrast of high vacuum, or penetrative.

A soft tube, as compared with a harder one—
(1) Permits passage of an electric current with a lower electro-motive force (E.M.F.) in the tube circuit;
(2) Produces grreater quantity of X rays;
(8) Emits rays which possess higher actinic power;
(4) Emits rays of lower penetration.

By penetration is meant the relative power of the rays produced to pass through objects interposed in their path.

This, rays from a soft tube will with difficulty penetrate bone; and if exposed to such rays the larger bones of the body will cast very deep shadows. On the other hand, a soft tube will reveal detail of structure in the smaller bones which would be entirely undefined by the more penetrating raya from a hard tube suitable for viewing denser structures.

Thus the degree of vacuum, with its concomitant power of penetration, will indicate the value of a tube for any specified purpose, and results will depend largely on the judicious choice of a tube. It is well, therefore, to have in use several tubes of varying degrees of hardness, and to let each be reserved for uses suitable to its special powers.

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