In the chapter on electricity in the atmosphere we saw that whereas air at ordinary pressure is a bad conductor, its conducting power increases rapidly as the pressure is lowered. Roughly speaking, if we wish to obtain a spark across a gap of 1 inch in ordinary air, we must have an electric pressure of about 50,000 volts. The discharge which takes place under these conditions is very violent, and it is called a “disruptive” discharge. If however the air pressure is gradually lowered, the discharge loses its violent character, and the brilliant spark is replaced by a soft, luminous glow.
The changes in the character of the discharge may be studied by means of an apparatus known as the “electric egg.” This consists of an egg-shaped bulb of glass, having its base connected with an air-pump. Two brass rods project into the bulb, one at each end; the lower rod being a fixture, while the upper one is arranged to slide in and out, so that the distance between the balls can be varied. The outer ends of the rods are connected to an induction coil or to a Wimshurst machine. If the distance between the balls has to be, say, half an inch, to produce a spark with the air at normal pressure, then on slightly reducing the pressure by means of the air-pump it is found that a spark will pass with the balls an inch or more apart. The brilliance of an electric spark is due to the resistance of the air, and as the pressure decreases the resistance becomes smaller, so that the light produced is much less brilliant. If the exhaustion is carried still further the discharge becomes redder in colour, and spreads out wider and wider until it loses all resemblance to a spark, and becomes a luminous glow of a purple or violet colour. At first this glow seems to fill the whole bulb, but at still higher vacua it contracts into layers of definite shape, these layers being alternately light and dark. Finally, when the pressure becomes equal to about one-millionth of an atmosphere, a luminous glow surrounds the cathode or negative rod, beyond this is dark space almost filling the bulb, and the walls of the bulb between the cathode and the anode glow with phosphorescent light. This phosphorescence is produced by rays coming from the cathode and passing through the dark space, and these rays have been given the name of “cathode rays.”
Many interesting experiments with these rays may be performed with tubes permanently exhausted to the proper degree. The power of the rays to produce phosphorescence is shown in a most striking way with a tube fixed in a horizontal position upon a stand, and containing a light cross made of aluminium, placed in the path of the rays. This is hinged at the base, so that it can be stood up on end or thrown down by jerking the tube. Some of the rays streaming from the cathode are intercepted by the cross, while others pass by it and reach the other end of the tube. The result is that a black shadow of the cross is thrown on the glass, sharply contrasted with those parts of the tube reached by the rays, and which phosphoresce brilliantly. After a little while this brilliance decreases, for the glass becomes fatigued, and loses to a considerable extent its power of phosphorescing. If now the cross is jerked down, the rays reach the portions of the tube before protected by the cross, and this glass, being quite fresh, phosphoresces with full brilliance. The black cross now suddenly becomes brilliantly illuminated, while the tired glass is dark in comparison. If the tired glass is allowed to rest for a while it partly recovers its phosphorescing powers, but it never regains its first brilliance.
An even more striking experiment may be made with a horizontal tube containing a tiny wheel with vanes of mica, something like a miniature water-wheel, mounted on glass rails. When the discharges are sent through the tube, the cathode rays strike against the vanes and cause the little wheel to move forward in the direction of the anode. Other experiments show that the cathode rays have great heating power, and that they are deflected by a magnet held close to the tube.
For a long time the nature of these cathode rays was in dispute. German physicists held that they were of the same character as ordinary light, while English scientists, headed by Sir William Crookes, maintained that they were streams of extremely minute particles of matter in a peculiar fourth state. That is to say, the matter was not liquid, or solid, or gaseous in the ordinary sense, but was ultra-gaseous, and Crookes gave it the name of radiant matter. Most of us have been taught to look upon the atom as the smallest possible division of matter, but recent researches have made it clear that the atom itself is divisible. It is believed that an atom is made up of very much more minute particles called electrons, which are moving about or revolving all the time with incredible rapidity. According to Sir Oliver Lodge, if we imagine an atom of hydrogen to be as big as an ordinary church, then the electrons which constitute it will be represented by about 700 grains of sand, 350 being positively electrified and 350 negatively electrified. It is not yet definitely determined whether these electrons are minute particles of matter charged with electricity, or whether they are actually atoms of electricity. The majority of scientists now believe that the cathode rays consist of a stream of negative electrons repelled from the cathode at a speed of 124 miles per second, or not quite 1/1000 of the velocity of light.
In November 1895, Professor Röntgen, a German physicist, announced his discovery of certain invisible rays which were produced at the same time as the cathode rays, and which could penetrate easily solids quite opaque to ordinary light. He was experimenting with vacuum tubes, and he found that certain rays emerged from the tube. These were not cathode rays, because they were able to pass through the glass, and were not deflected by a magnet. To these strange rays he gave the name of the “X,” or unknown rays, but they are very frequently referred to by the name of their discoverer.
It was soon found that the Röntgen rays affected an ordinary photographic plate wrapped up in black paper so as to exclude all ordinary light, and that they passed through flesh much more easily than through bone. This fact makes it possible to obtain what we may call “shadow-graphs” of the bones through the flesh, and the value of this to the medical profession was realized at once. The rays also were found to cause certain chemical compounds to become luminous. A cardboard screen covered with one of these compounds is quite opaque to ordinary light, but if it is examined when the Röntgen rays are falling upon it, it is seen to be brightly illuminated, and if the hand is held between the screen and the rays the bones become clearly visible.
Röntgen rays are produced when the cathode rays fall upon, and as it were bombard, an obstacle of some kind. Almost any tube producing cathode rays will produce also Röntgen rays, but special forms of tube are used when the main object is to obtain these latter rays. Fig. 38 shows a typical form of simple X-ray tube. This, like all other tubes for X-ray work, is exhausted to a rather higher vacuum than tubes intended for the production of cathode rays only. The cathode C is made of aluminium, and is shaped like a saucer, its curvature being arranged so that the cathode rays are focused on to the anti-cathode A. The focusing as a rule is not done very accurately, for although sharper radiographs are obtained when the cathode rays converge exactly to a point on the anti-cathode, the heating effect at this point is so great that a hole is quickly burned. The target, or surface of the anti-cathode, is made of some metal having an extremely high melting-point, such as platinum, iridium, or tungsten. It has a flat surface inclined at an angle of about 45°, so that the rays emanating from it proceed in the direction shown by the dotted lines in the figure. The continuous lines show the direction of the cathode rays. The anode is made of aluminium, and it is shown at N. It is not necessary to have a separate anode, and the anti-cathode may be used as the anode. In the tube shown in Fig. 38 the anode and the anti-cathode are joined by an insulated wire, so that they both act as anodes. The tube is made of soda-glass, as the X-rays do not pass at all readily through lead-glass.
By permission of]
[C. H. F. Muller.
Fig. 39.—Diagram of Mica Vacuum Regulator for X-Ray Tubes.
The penetrating power of the X-rays varies with the vacuum of the tube, a low vacuum giving rays of small penetration, and a high vacuum rays of great penetration. Tubes are called hard or soft according to the degree of the vacuum, a hard tube having a high vacuum and a soft tube a low one. It should be remembered that the terms high and low, as applied to the vacuum of X-ray tubes, are only relative, because the vacuum must be very high to admit of the production of X-rays at all. The vacuum becomes higher as the tube is used, and after a while it becomes so high that the tube is practically useless, for the penetrating power of the rays is then so great that sharp contrasts between different substances, such as flesh and bone, cannot be obtained, and the resulting radiographs are flat and poor. The vacuum of a hard tube may be lowered temporarily by gently heating the tube, but this is not a very convenient or satisfactory process, and tubes are now made with special arrangements for lowering the vacuum when necessary. There are several vacuum-regulating devices, and Fig. 39 is a diagram of the “Standard” mica regulator used in most of the well-known “Muller” X-ray tubes. This consists of a small additional bulb containing an electrode D carrying a series of mica discs. A wire DF is attached to D by means of a hinged cap. The vacuum is lowered while the discharges are passing through the tube. The wire DF is moved towards the cathode terminal B, and kept there for a few seconds. Sparks pass between F and B, and the current is now passing through the electrode D in the regulator chamber. This causes the mica to become heated, so that it gives off a small quantity of gas, which passes into the main tube and so lowers the vacuum. The wire DF is then moved well away from B, and after a few hours’ rest the tube, now of normal hardness, is ready for further use.
We have already referred to the heating of the anti-cathode caused by the bombardment of the cathode rays. Even if these rays are not focused very sharply, the anti-cathode of an ordinary tube becomes dangerously hot if the tube is run continuously for a fairly long period, and for hospital and other medical work on an extensive scale special tubes with water-cooled anti-cathodes are used. These tubes have a small bulb blown in the anti-cathode neck. This bulb is filled with water, which passes down a tube to the back of the target of the anti-cathode. By this arrangement the heat generated in the target is absorbed by the water, so that the temperature of the target can become only very slightly higher then 212° F., which is the temperature of boiling water, and quite a safe temperature for the anti-cathode. In some tubes the rise in temperature is made slower by the use of broken bits of ice in place of water. Fig. 40 shows a Muller water-cooled tube, and Fig. 41 explains clearly the parts of an X-ray tube and their names.
By permission of]
[C. H. F. Muller.
Fig. 41.—Diagram showing parts of X-Ray Tube.
An induction coil is generally used to supply the high-tension electricity required for the production of the Röntgen rays. For amateur or experimental purposes a coil giving continuous 4-inch or even 3-inch sparks will do, but for medical work, in which it is necessary to take radiographs with very short exposures, coils giving sparks of 10, 12, or more inches in length are employed. An electrical influence machine, such as the Wimshurst, may be used instead of an induction coil. Very powerful machines with several pairs of plates of large diameter, and driven by an electric motor, are in regular use for X-ray work in the United States, but in this country they are used only to a very small extent. A Wimshurst machine is particularly suitable for amateur work. If a screen is to be used for viewing bones through the flesh a fairly large machine is required, but for screen examination of such objects as coins in a box, or spectacles in a case, and for taking radiographs of these and other similar objects, a machine giving a fairly rapid succession of sparks as short as 2 inches can be used. Of course the exposure required for taking radiographs with a machine as small as this are very long, but as the objects are inanimate this does not matter very much.
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
Kodak Ltd.
RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS. TAKEN ON WRATTEN X-RAY PLATE.