Man Sees Through Solids
Wilhelm Conrad Rontgen was Professor of Physics and Director of the Physical Institute in Wurtzburg in Germany. He was an impetuous man with little time for the complicated apparatus in which many of his colleagues indulged. For real research, a physicist should be able to make anything he needed, with his own pocket-knife, little else.
In the autumn of 1895 he embarked on a series of experiments. True to his principles, he had made the apparatus himself, in his laboratory, though perhaps with the occasional help of tools more advanced than a pocket-knife. There was the large coil of wire, wound by his own hands, thousands upon thousands of turns of fine wire, which could raise the pressure, the voltage, of electric current from his home-made battery; there was his own version of the vacuum-tube invented in England by Sir William Crookes. This was a piece of glass blown into the size and shape of a vegetable marrow, with metal electrodes sealed into it. The glass was emptied of air by a pump, also homemade, and then Rontgen connected up his battery, through the coil, to the two electrodes, whose ends projected outside the tube.
Crookes had shown that a stream of electrons could be attracted away from the negative electrode, in such a vacuum, made to hop across a considerable space to the positively charged electrodes some distance away. The two electrodes, negative and positive, had been given, respectively, the names “cathode” and “anode”. The behaviour of these electrons, “cathode rays” as many now called them, was fascinating to scientists. Among other things, the rays had the property of making certain substances glow in the dark, if they chanced to hit them. But as they seemed able to move only in a vacuum, no one could think of a practical application. Rontgen switched on his apparatus as usual. Then, to his astonishment, he noticed a large crystal, which he had absent-mindedly left on a corner of his workbench, light up as if someone had lit a small fire inside. It was a crystal of one of the substances which, placed against the glass of a Crookes tube, could be made to glow feebly. Now it was shining brightly, a foot away from the tube.
Puzzled by this, Rontgen tried sealing the tube in thick black paper; the crystal still glowed, just as brightly.
Next he ground up the crystal in his mortar, spread the grains on adhesive-covered paper. The paper, when he held it upright, glowed like a sunlit window.
Cathode rays, which were believed, as electrons, to be solid particles, could never have behaved like this; Rontgen decided he had struck a new form of radiation, some wave-form, like the electro-magnetic waves of light and heat. He was a careful man in this sort of thing, knew that his research into the phenomenon was just beginning, and he called his discovery, simply, “X-rays”, the unknown quantity. The name has stuck, though now we know a great deal about them. In some parts of the world, in honour of their discoverer, they are called Rontgen Rays.
Wilhelm Rontgen soon found that they had the interesting characteristic of being easily blocked by certain objects, not at all by others. The bones of the hand, for example, seemed to block the rays entirely, yet they passed through the surrounding flesh as if it were not there. He discovered that they exposed photographic film, just as did visible light, and now if he put his hand on a covered photographic plate (covered, so light would not reach it) and directed X-rays on it, he found, when he removed the covering, a picture of the hand on the photographic plate. But it was not the usual hand, the one he knew; it was the hand of a skeleton. The bones were clearly visible as white lines, the flesh around them grey, the rest of the plate black. A ring on his finger showed as a white band.
This seemed to Rontgen to hold some possibility for the medical profession, though he doubted if it would be possible to take useful photographs of bodily organs. Bones, yes, but not organs, which had much the same density as the flesh around them.
In fact, it was soon possible to investigate almost every part of the body with the aid of these X-rays. As machines for making the rays grew better, more efficient, than Rontgen’s home-made coil and tube, and as the quality and speed of the film was improved, it became possible to study the condition of, say, the lungs, detect traces of injury, abnormality. The heart in the middle of its sharply defined rib cage could be seen clearly; any alteration in its size could be noted. The stomach and intestines presented problems, because their walls were thin and offered so little resistance to the X-rays.
Then someone persuaded an unfortunate guinea-pig to drink a solution of powdered lead and found that the outline, the shape, of the animal’s digestive tract was clearly shown in an X-ray photo: the rays were blocked by the lead.
Lead is poisonous to human beings, and to guinea-pigs, but investigators found that barium and bismuth, which are not, served the same purpose. Now, before any X-ray examination of the stomach or intestine, the patient is given a “barium meal” which delineates the whole of the digestive tract, showing any blockages.
By the principle of Rontgen’s sheet of paper with powdered, “fluorescent” crystal glued to it, we can now, if we wish, view the organs of the body without help from a camera. If X-rays are passed through the body and allowed to strike the back of such a fluorescent sheet (made nowadays of glass) we can stand in front and observe, turn the patient sideways, back to front, get the clearest view of what we want to see.
Rontgen proved that X-rays were not identical with cathode rays, were produced by them, a secondary effect, when they hit the wall of the Crookes tube or bounced off its positive electrode. By designing this “anode” so that it presented an oblique surface to the stream of electrons from the cathode, it became possible to bounce very powerful X-rays outside the tube, at right-angles to the electron flow. If a second anode were placed behind the first, adding to its attraction, the quantity of X-rays could be greatly increased.
Modern X-ray tubes work on the same principle, but provide a source of electrons independent of the high voltage between cathode and anode; a steady, controllable, source from a small spiral of tungsten wire, heated by a small current. A constant stream of electrons, as in a radio valve, is provided by this hot metal, and the actual quantity can be regulated by altering its heat. The force of the radiation, its penetrating power, can be altered by varying the very high voltage, an electrical pressure of anything up to 100,000 volts, between this hot cathode and the positively charged anode at the other end of the tube.
X-rays, as Rontgen maintained, are electro-magnetic waves, identical in character with light, but of far shorter wave-length. Their peculiar property is caused by the fact that they are so small they can squeeze between the individual atoms of most substances. Visible light, being “larger”, cannot do so. Apart from their immensely valuable role in releasing secrets of the body’s interior, the interior of anything, X-rays have important applications in therapy. They are widely used in the treatment of cancers because it has been found that they damage, destroy, the cells of the body, but tend to affect malignant, cancerous, cells much more than healthy ones; a convenient and valuable phenomenon. Apart from their use against cancer, X-rays have been found to be highly effective against certain skin conditions like ringworm.
The most deeply penetrating X-rays are those of the shortest wave-length, and these are essentially the same as the “gamma rays” emitted by radium. Both sorts are used in therapy and to a large extent they are interchangeable. The same unit of dosage, the “Rontgen”, is used for both. It has now been found that radium is by no means the only “radio-active” substance: others can be made radio-active, emitting radiations at varying rates and of varying power, and these have opened up whole new fields of research. Radio-active substances can be used, not only to make hydrogen bombs, but to investigate the human body, to trace industrial processes from start to finish, detect errors.
Radioactive cobalt, a highly lethal substance, is now being widely used in radiation therapy, and the original X-ray tube of Wilhelm Rontgen has been so improved that rays of a penetrating power undreamed of only a few years back are now freely available.
The most rapidly expanding application of X-rays is in industry. “Whenever it is necessary to investigate the structure of a finished article without damaging it, X-rays are used. Hidden fractures in metal castings can be spotted instantly, and the items rejected before any expensive further work is done on them. Faulty golf-balls (the X-ray machine can tell us at last whether they or we are responsible for a bad round), bad glass, fake gems, all can be detected by the camera. Paintings which have been altered, bogus “Old Masters” are spotted instantly under its penetrating gaze.
Unfortunately, none of the early experimenters with X-rays, or with radium, realized that exposure of the skin to them would produce, over a period of time, such dangerous burns that even cancer, the disease it was hoped the rays might conquer, could result. Many of the pioneers of X-ray succumbed to cancer and many who survived had to have fingers, hands amputated. The effect of the rays was slow, misleadingly slow, and men and women went on working unprotected for years before the first symptoms appeared, by which time it was too late. The same happened with the pioneer of radium, Madame Curie, who died of “radium poisoning”.
At the outbreak of war in 1914, Wilhelm Rontgen was Professor in Munich. He was ennobled at about this time, by the Bavarian Government, was able to use the coveted “Von” before his name, a distinction corresponding to an English baronetcy, but his health and that of his wife, famous because his X-ray photo of her hand, far better than the accidental one he made of his own, is the first preserved X-ray photograph, began to suffer. He was an ardent patriot, prayed nightly that God would destroy the enemies of Germany, but somehow seemed unwilling to take any practical steps to help Him. In France, Marie Curie was driving her X-ray van furiously a few miles behind the front line, using one of Rontgen’s own tubes to investigate wounds, performing miracles not only for French soldiers but for German prisoners as well; but by now Rontgen was an old, tired man. He became a complete recluse. He had obtained his share of the world’s honours, they included the Nobel Prize for his discovery, in 1901; he was content to let others use, develop, his rays.
He survived the war, still a recluse, shunning any sort of publicity, but quietly working on important research, the electro-magnetic rotation of polarized light, the ratios of the specific heats of gases, the conduct of heat through crystals, until he died in 1923.
Rontgen’s discovery has revolutionized so much that we can hardly list it. Medicine and surgery were the first sciences to feel its impact, but within a few years this discovery of a lump of crystal glowing on a work-bench had upset the whole of physics. It was not until Albert Einstein and his even more revolutionary theory of Relativity that it received another such shock. Hospitals, laboratories, factories now gleam with huge successors to the original Rontgen equipment, so painstakingly made by hand, and though their performance is far superior to the first feeble burst of X-rays in October, 1895, that experiment lit up whole new regions for man to explore.
- Historical X-ray tubes
- Röntgen’s 1895 article, on line and analyzed on BibNum [click ‘à télécharger’ for English analysis]
- Example Radiograph: Fractured Humerus
- A Photograph of an X-ray Machine
- X-ray Safety
- An X-ray tube demonstration (Animation)
- 1896 Article: “On a New Kind of Rays”
- “Digital X-Ray Technologies Project”
- What is Radiology? a simple tutorial
- 50,000 X-ray, MRI, and CT pictures MedPix medical image database
- Index of Early Bremsstrahlung Articles
- Extraordinary X-Rays – slideshow by Life
- X-rays and crystals