The Crookes Tube.
The most satisfactory tube for X-ray work is one where the vacuum is readily adjustable. Reference to Fig. 73 shows the Queen form. A small bulb, containing a chemical which gives off vapor when heated and reabsorbs it when cooled, is directly connected to the main tube and surrounded by an auxiliary tube, which is exhausted to a low vacuum. In the auxiliary tube the cathode is opposite to the above-mentioned bulb, so that any discharge through it will heat the bulb by the bombardment of the cathode rays. The cathode is connected to a spark point, which can be adjusted to any distance from the cathode of the main tube. The anode of the small tube is directly connected to that of the main tube. When the tube is put into operation the vacuum and, consequently, the resistance of the main tube being high, the current preferably passes by the spark point and auxiliary tube, heating the chemical for a few seconds until sufficient vapor has been driven into the main tube to permit the current to pass through the latter. After this only an occasional spark will jump across the gap to counteract the tendency of the reabsorption of the vapor and consequent raising in resistance of the main tube.
This device presents easy means of adjusting the vacuum in the main tube. With the spark point at a considerable distance from cathode the vacuum will be high. When the spark gap is short the vacuum will become low. The main bulb is about 4½ inches in diameter, and at the place where the X-rays pass only 1∕64 of an inch in thickness. The cathode is of aluminum, the anode of platinum. In starting this tube, it is best to make the spark gap about one inch in width. When connected up and working properly the main bulb will be filled with a green striated luminosity between anode and cathode, and the tip of the chemical bulb will have the shadow of the little platinum tip thrown upon it. The green light is not always brilliant; at times it is quite weak, but yet does its work well. A brilliant green light is often one of the signs of wrong connection, and particularly so when the little shadow on the chemical bulb is absent. Never run these or any other tubes backwards, but be sure the current is flowing in correct direction at first operation.
Other forms of Crookes tubes differ only in form, or are devoid of adjustment, and the connections of coil, tube, etc., are the same.