The condition of the tube is ascertained by finding its “equivalent spark gap.” While the coil and tube are working, the terminal points of the induction coil are slowly brought together. If a spark passes between the points while they are six inches or more apart, the vacuum is too high. If no sparking takes place between the terminals till they are within three inches of each other, the tube is low. A good working spark gap distance is four and one-half inches. A soft, or low-vacuum, tube gives better definition than a hard, or high-vacuum, tube, as the rays pass less easily through dense substances and show greater differentiation of tissue. A very high vacuum tube may show but little difference between the bones and flesh, while a soft tube should give the minute structure of the bones.
Time of Exposure.—With a current of five amperes at one hundred volts passing through the primary winding of a ten-inch coil, the exposure for a hand or foot would be from three to fifteen seconds. The exposure for the thicker portions of the body would be from twenty seconds to two minutes. If an electrolytic break is used, about half the exposure would be required. Dry plates with extra thick sensitive films are specially prepared for radiography, the development and fixation being the same as in ordinary photography. The image is sometimes barely visible on the surface of the plate during development, but when fixed the negative may give good density and definition owing to the penetration into the film of the X-rays.
Kinds of X-rays.—It is now known that these rays are not all by any means of the same kind or of the same penetrative power. Moreover, these differences can be still augmented by altering what is known as the induction in the circuit, the degree of exhaustion in the tube, and the nature of the emitting surface. The emitting surface is not the glass walls of the tube, as many suppose; and the canary colored light emitted by the tube is not the X-rays, which are themselves invisible. They originate from the anode of the tube owing to the fierce bombardment to which the cathode rays subject it. Where the cathode rays, which travel in straight lines, first strike any material object, from that same object the X-rays originate.
Uses of X-rays.—In the early days of radiography the X-rays in medical work were confined almost solely to the detection of fractured or injured bones, and abnormal bone growth. At the present time, however, even a careful examination on the fluorescent screen is sufficient to enable an expert medical radiographist to diagnose with a considerable degree of exactitude the condition of the heart, the lungs, and the stomach. In making such examination a tube must be chosen which has the lowest vacuum, in order to obtain the maximum amount of contrast between fleshy tissue not differing greatly in density.
In some cases even the liver has been outlined and part of the kidneys.
Still more important is the fact that the rays have been applied successfully in the treatment of certain diseases which by other means have been deemed, if not incurable, at any rate extremely difficult to cure. Claims have been made for cancer cures by means of these same rays; whether these have really been complete cures or not is perhaps open to question.
X-ray Dermatitis.—A painful and incurable disease, of a cancerous nature, to which radiographers are liable, caused by frequent and prolonged exposure to X-rays. Many of the pioneers of radiography have fallen victims to this complaint, but greater precautions are now taken to protect the operators from the X-rays. There is little danger of contracting this disease in X-ray photography, as the exposures are short and the operator need not stand directly in front of the tube. The chief risk is entailed by visual examination with the fluorescent screen. The disease first makes its appearance in the hands and gradually spreads to the arms and body. The skin at first appears as if it had been burned, hence the term “X-ray burning.”
THE LIGHT THAT REVEALS THE UNSEEN IN THE HUMAN BODY
