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.
General Remarks.
A high vacuum gives greater penetrative power than a low vacuum. Where the operator has not an adjustable tube it is imperative that he have at least two tubes, one high and one low. It is the contrasts which render the X-ray practical, and these contrasts are largely governed by the vacuum. In locating a metallic substance in the human body a high vacuum tube would be needed, that the bones and dense tissue be rendered more transparent. On the other hand, to make a radiograph of the bones, a lower vacuum is necessary in order to get a contrast between the bones and the tissues. In general, a high vacuum is best for fluoroscope work and a low vacuum for making pictures on a photographic plate. Short exposures in radiography are obtained by powerful rays and consequently by coils operating at considerable energy. In extended examinations or where a subject is under the X-rays for more than a minute or so, a screen should be interposed between the subject and the tube to avoid the burning effect which is often noticeable. This screen consists of a piece of cardboard well covered with gold leaf, and should be grounded—that is, a connection be run from the gold surface to a water-pipe or other ground connection. Sheet lead is an efficient screen to the rays, and, if desired, a lead screen can be made, partially enclosing the apparatus, to protect the operator. But it must be large enough and far enough distant from the coil and tube to avoid any possibility of leakage of current or even inductive influence. In operating X-ray machines never attempt to alter connections or make adjustments other than at coil platinum screw or Crookes tube spark gap without first shutting off current. Remember that a very unpleasant shock can be easily obtained from touching the apparatus with only one hand. It is often advisable to remove one's watch, particularly when using Ruhmkorff coils of large size.
The tube may be worked until it shows a slight redness in the centre of the platinum, but care must then be taken not to increase current, or the platinum will melt. Never allow the tube to come in contact with any object other than its stand and connections while working, and be sure the wires from secondary do not come near tube until they reach places of attachment, or they may spark through glass and ruin the tube.
In making radiographs on sensitized plates the unused plates should be kept at a considerable distance from the coil while working. Better still if they are in another room. Plates for X-ray work are made by most photographic supply dealers; in fact, almost any good brand of sensitized plates or even films will answer. When making a radiograph, the plate can either be left in the holder or well wrapped in black paper, but current should never be turned on coil before the plate and subject are in position. In photographing the chest, neck, etc., the plate can be strapped on to the part; but the subject must remain absolutely still. The time of exposure varies considerably with the size of coil, thickness of object, etc. Radiographs of the hand have been taken by simply laying the hand on top of the plateholder and operating tube for 100 seconds. But, as a rule, longer exposures are necessary. Most radiographs will generally require that the plate be "intensified" and a developer used that gives great detail, such as metol quinol, etc. At any rate, great care should be exercised in developing the plate, as many a good radiograph has been spoiled by undue haste.