GENERAL PRINCIPLES OF RADIOGRAPHY 924. X-ray Generators. X-rays (W. C. Röntgen, 1895) are comparable with rays of light, but have wavelengths shorter than those of the extreme ultra-violet, and are produced by a very different process.
An X-ray generator, reduced to its simplest form, consists of a thin bulb of high-melting point glass, fitted with two metal electrodes and evacuated. When these two electrodes are connected to the two poles of a source of electricity (continuous or rectified current) which have a considerable difference of potential (4o,000 to ioo,000 volts), the cathode, i.e. the electrode connected to the negative pole, emits a stream of very small particles of electricity, electrons, at right angles to its surface ; these are projected at a very high velocity.
The wall of the tube, or preferably the other electrode (the anti-cathode), which as a rule is inclined at an angle of 45° to the stream of cathode rays, is bombarded by this stream and becomes the source of the emission in all direc tions of the X-rays. If the cathode is made in the form of a spherical cup whose centre of curvature is situated on the surface of the anti-cathode, the cathode rays, converging on to the centre of curvature, will form a focus of X-ray emission of very small size approaching a point ; but the overheating of the anti-cathode is then enormous.' X-ray generators have been greatly improved by an invention of W. D. Coolidge (1913). In the Coolidge tube the vacuum is such that even with a potential difference of 200,000 volts no discharge occurs, nor are any electrons The cathodic bombardment cannot occur until the cathode is made incandescent ; this is achieved by making the cathode in the form of a spherically coiled cap of tungsten wire through which an auxiliary heating current flows (8 to 12 volts, o'5 to Po amp.). The speed of the emission of the electrons depends entirely on the difference of potential between the cathode and anti-cathode, but their number, and in consequence the ntensity of the cathode stream, may be regulated at will by the temperature of the cathode, that is to say, by the intensity of the heating current. There is thus avoidance of
the irregularities, due to the variations in the pressure of the residual gas in the older type of tubes, which were much less completely evacu ated and are now known as gas tubes.
A further advantage of the incandescent cathode type of tube accrues from the dissimi larity of the two electrodes ; the anti-cathode is constantly maintained at a temperature lower than that necessary for the emission of electrons, and consequently the current is at once arrested if it becomes reversed. Hence these tubes will function, without valves or rectification devices, on a high-voltage source of alternating current, e.g. from a transformer.
The anti-cathode of these generators consists of a plate of tungsten mounted at the end of a copper rod extending to the exterior of the tube in the form of a sleeve provided with blades which act as a radiator for cooling the anti-cathode.' The Coolidge tube will work for a considerable time with perfect regularity ; the quality and the intensity of the X-rays can be varied at will ; the limiting factor is the possibility of melting the anti-cathode and thus putting the tube out of action. In order to avoid the risk of destroying so expensive a piece of apparatus, it is recommend ed that the current should be switched off when ever the anti-cathode reaches a dull red heat.
925. Properties of X-rays. X-rays produce no visual effect ; the greenish light emitted by gas tubes is a fluorescence of the glass caused by cathodic bombardment and is not connected with X-rays. X-rays can be observed and used by their action on a photographic emulsion (radiography), and by the fluorescence which they excite in many substances, e.g. barium platinocyanide (viewing screens) and calcium tungstate (intensifying screens for radiography).