Modifications of X-Ray Tubes.

I. Addition of a third electrode is one of the earliest and most common modifications of the tube from the form designed by Jackson. This serves as an anode, placed axially opposite to the kathode, and is connected outside by a wire to the target antikathode, the stem of which passes through the tube wall in a line at right angles to the desired plane of the target.

This arrangement is said to steady the action of the tube in a manner variously explained, and also to prolong its working life by retarding the change in quality described above, but its service is questionable. In France and America it seems to be little used, though in the German pattern of tube, which is mainly followed in this country, it is almost universal.

II. A larger diameter is given to tubes than formerly, changes temporary and permanent being thus delayed, and the life of the tube prolonged. Originally tubes of 2£ inches in diameter were commonly used; now 5 or 6 inches is a common diameter; whilst tubes are made of 9 or 10 inches diameter, and may become more general.

Before choosing a tube of large diameter, however, one must ascertain whether the internal electrodes of the tube are set correspondingly farther apart, for this distance may be limited by the power of the coil employed. With a small coil say a 10-inch size—a tube with electrodes set far apart might soon become too resistant for the full power of the coil to excite, while a tube of smaller proportions under similar circumstances would permit excitation and emit rays of sufficient penetration for all purposes.

With tubes of proportions as ordinarily made we would suggest that—

With a 10-inch coil, a tube be used of 13 centimetres or 5 inches in diameter;

With a 15-inch coil, a tube be used of 15 centimetres or 6 inches in diameter.

With static machines smaller tubes are more serviceable, one of 8 centimetres or 3 inches diameter giving useful penetration.

Of coarse, makers could construct larger bulbs while main- taining the shorter distance between the electrodes; but the two dimensions usually vary in proportion,.hence the above caution should be remembered. …… .

III. A vacuum regulator of some kind is added to almost all modern tubes, except the very low-priced ones. Such an addition is an ultimate economy, since it counteracts the hardening effect of continued use, and thereby prolongs the period of usefulness of the tube.

These regulators, when brought into action, give off, or transmit, gaseous substance, whereby the number of elec- trons in the interior of the X-ray tube is increased, and the degree of vacuum correspondingly reduced. They are set in action by heat, produced either by electric discharge or by direct application of a flame to the regulator.

(a) A small side-tube containing a chemical (such as KHO), which gives off vapour when heated, may be attached in construction to the X-ray tube. When desired, this is heated by a flame applied to the outside, and gas is thereby driven off, which passes into the main tube. This early form is now superseded by more con- venient arrangements, but illustrates the elementary principle of such regulators.

(b) A side-tube with some capillary substance, such as woven glass, mica discs, asbestos, or spongy metal, may be similarly connected to the main tube, as shewn in Fig. 7. The capillary substance is arranged at one end of the side-tube, and is traversed or surrounded by a platinum electrode, to which, when put in circuit, sparks may discharge from an electrode in the form of an aluminium disc at the kathode end of the tube. By such discharge heat is generated, and the gaseous contents of the capillary substance are thereby caused to expand, and part to be expelled into the vacuum of the larger tube. . .

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