[Illustration: FIG. 1.--The evolution of the Crooke's tube.]
The manufacture of the X-ray tubes tests technical skill and the patience of the experimenter more highly, perhaps, than the preparation of any apparatus used in science. Glass working is a difficult art, and requires an absolute devotion to it. There is only one metal known which will enable an electrical discharge to pass into and out of a rarefied space inclosed by glass. This is platinum. A wire of this metal can be sealed into glass so that no air can leak into an exhausted space around the joints. All electric lamps, so commonly used in electric lighting, have little wires of platinum at their bases, by means of which the electric current enters and leaves the bulb. The Crooke's tube is in principle an Edison lamp with the filament broken. The maker of Crooke's tubes should complete the making of the tube at one sitting, for reheating of the tube is very apt to lead to a disastrous cracking of the glass. He must take the utmost precautions against unequal heating and sudden cooling, and he must, above all, have phenomenal patience.
Fig. 1 shows the evolution of the Crooke's tube which is used to produce the X rays. The first form of tube was barely larger than a goose's egg. The size has been gradually increased, and at present it is three or four times larger than the original form. The interior arrangement has not been materially changed, and consists, as we have said, of a concave mirror, which constitutes the negative electrode, and an inclined sheet of platinum, from which the X rays seem to emanate.
The later forms of tube have accessory chambers, filled with certain chemicals, which, on being slightly heated, reduce the vacuum to the desired point. Certain forms of tubes have merely an additional chamber which, on being heated, reduces the vacuum in the main vessel. The latest form of tube, devised by Dr. William Rollins, of Boston, has a hollow anode tube (_B C_, Fig. 1), through which a current of water can circulate in order to save the tube from breaking. The end of this anode tube is small, in order to form a sharp radiant point of light. One of the platinum wires (_P_) inserted in the tube projects outside some distance. When the vacuum becomes too high in the tube, this platinum wire is slightly heated in a gas flame; then the flame is blown out and the hydrogen is allowed to flow against the heated wire. A sufficient amount of the gas is absorbed by the heated wire to reduce the vacuum in the tube. This tube stands very powerful electrical discharges, and is the most scientifically designed tube at the command of the experimenter.