For amateur X-ray work the arrangement of the apparatus is simple. The tube is held in the required position by means of a wooden clamp attached to a stand in such a way that it is easily adjustable. Insulated wires are led from the coil or from the Wimshurst machine to the tube, the positive wire being connected to the anode, and the negative wire to the cathode. With a small Wimshurst machine light brass chains may be used instead of wires, and these have the advantage of being easier to manipulate. For medical purposes the arrangements are more complicated, and generally a special room is set apart for X-ray work.
If the connexions have been made correctly, then on starting the coil or the machine the tube lights up. The bulb appears to be sharply divided into two parts, the part in front of the anti-cathode glowing with a beautiful greenish-yellow light, while the part behind the anti-cathode is dark, except for lighter patches close to the anode. The Röntgen rays are now being produced. The illumination is not steady like that of an electric lamp, but it consists of a series of flickers, which, with powerful apparatus, follow one another so rapidly as to give the impression of continuity. If the connexions are wrong, so that the negative wire goes to the anode instead of to the cathode, the bulb is not divided in this way, but has patches of light almost all over. As soon as this appearance is seen the apparatus must be stopped and the connexions reversed, for the tube is quickly damaged by passing the discharge through it in the wrong direction.
Having produced the X-rays, we will suppose that it is desired to examine the bones of the hand. For this purpose a fluorescent screen is required. This consists of a sheet of white cardboard coated usually with crystals of barium platino-cyanide. In order to shut out all light but that produced by the rays, the cardboard is placed at the larger end of a box or bellows shaped like a pyramid. This pyramid is brought close to the X-ray tube, with its smaller end held close to the eyes, and the hand is placed against the outer side of the cardboard sheet. The outline of the hand is then seen as a light shadow, and the very much blacker shadow of the bones is clearly visible. For screen work it is necessary to darken the room almost entirely, on account of the feebleness of the illumination of the screen.
If a radiograph of the bones of the hand is to be taken, a very sensitive photographic plate is necessary. An ordinary extra-rapid plate will do fairly well, but for the best work plates made specially for the purpose are used. The emulsion of an ordinary photographic plate is only partially opaque to the X-rays, so that while some of the rays are stopped by it, others pass straight through. The silver bromide in the emulsion is affected only by those rays which are stopped, so that the energy of the rays which pass through the emulsion is wasted. If a plate is coated with a very thick film, a larger proportion of the rays can be stopped, and many X-ray plates differ from photographic plates only in the thickness of the emulsion. A thick film however is undesirable because it makes the after processes of developing, fixing, and washing very prolonged. In the “Wratten” X-ray plate the emulsion is made highly opaque to the rays in a different and ingenious manner. Salts of certain metals have the power of stopping the X-rays, and in this plate a metallic salt of this kind is contained in the emulsion. The film produced in this way stops a far larger proportion of the rays than any ordinary film, and consequently the plate is more sensitive to the rays, so that shorter exposures can be given.
X-ray plates are sold usually wrapped up separately in light-tight envelopes of black paper, upon which the film side of the plate is marked. If there is no such wrapping the plate must be placed in a light-tight envelope, with its film facing that side of the envelope which has no folds. The ordinary photographic double envelopes, the inner one of yellow paper and the outer one of black paper, are very convenient for this purpose. The plate in its envelope is then laid flat on the table, film side upwards, and the X-ray tube is clamped in a horizontal position so that the anti-cathode is over and pointing towards the plate. The hand is laid flat on the envelope, and the coil or machine is set working. The exposure required varies so much with the size of the machine or coil, the distance between the tube and the plate, the condition of the tube, and the nature of the object, that it is impossible to give any definite times, and these have to be found by experiment. The hand requires a shorter exposure than any other part of the body. If we call the correct exposure for the hand 1, then the exposures for other parts of the body would be approximately 3 for the foot and the elbow, 6 for the shoulder, 8 for the thorax, 10 for the spine and the hip, and about 12 for the head. The exposures for such objects as coins in a box are much less than for the hand. After exposure, the plate is developed, fixed, and washed just as in ordinary photography. [Plate XIV]. shows a Röntgen ray photograph of a number of fountain pens, British and foreign.
Prolonged exposure to the X-rays gives rise to a painful and serious disease known as X-ray dermatitis. This danger was not realized by the early experimenters, and many of them contracted the disease, with fatal results in one or two cases. Operators now take ample precautions to protect themselves from the rays. The tubes are screened by substances opaque to the rays, so that these emerge only where they are required, and impenetrable gloves or hand-shields, aprons, and face-masks made of rubber impregnated with lead-salts are worn.
X-ray work is a most fascinating pursuit, and it can be recommended strongly to amateurs interested in electricity. There is nothing particularly difficult about it, and complete outfits can be obtained at extremely low prices, although it is best to get the most powerful Wimshurst machine or induction coil that can be afforded. As radiography is most likely to be taken up by photographers, it may be well to state here that any photographic plates or papers left in their usual wrappings in the room in which X-rays are being produced are almost certain to be spoiled, and they should be placed in a tightly fitting metal box or be taken into the next room. It is not necessary for the amateur doing only occasional X-ray work with small apparatus to take any of the precautions mentioned in the previous paragraph, for there is not the slightest danger in such work.
PLATE XIV.
By permission of