by Nikola Tesla
December 2nd, 1896
The following lines may contain some useful information for physicists and physicians. Those who, in the exercise of their professional duties are applying the discoveries of Roentgen to the relief of the suffering by determining the position of foreign objects or ascertaining the condition of local troubles of malformations in the organism, are apt to be disappointed in many instances. While it is perfectly easy to find the position of a foreign object in the head, neck and all soft tissues of the body, and detect some far gone trouble in the lungs, often the location of even such a large and opaque object as a bullet, when imbedded in certain bony parts of the trunk of the patient, may be attended with difficulties. Success will be invariably attained if the suggestions which are given below, and which are the outcome of a number of observations of such cases, are strictly followed.
In order to make the present statements self-contained and more useful, I deem it of advantage to say a few words in regard to the Roentgen rays. According to all evidences so far obtained by me, I entertain the view, which I have expressed on other occasions, that these rays are formed by streams of some matter projected with great velocity, and generally intermittently, from the walls of the tube. The intermittent character is only due to this feature of the apparatus usually employed for the production of the rays; but the oscillatory or intermittent discharge is not absolutely necessary, as I have produced unidirectional currents of high tension which are likewise capable of generating strong rays, and as a static machine may be used with a like result. The mode of formation of these rays or streams is, for the present purposes, of little importance. The small particles within the bulb, which are the original cause, may be ions, formed by a process of electrolysis, or they may be comparatively larger particles of the electrode, or perhaps molecules of the residual gas. At any rate, it is probable that the particles are very minute, and that, therefore, the velocities of the cathodic streams within the vessel are such and the impacts so violent as to cause a further disintegration of the cathodic matter to state probably never before studied by physicists. We may have to deal, as I have already suggested, with an actual breaking up of the ether-vortexes, which, according to Lord Kelvin’s theory, compose the material particles, or we may be confronted with a dissolution of matter into some unknown primary form, the Akasa of the old Vedas. Experiments show that this matter is reflected, sometimes very well, sometimes poorly; but in all cases the various metals behave in a curious manner, which I have studied, and the results obtained, though probably not free of error, because of the great difficulties in getting an exact estimate in such an investigation, were, nevertheless, sufficiently positive as to lead me to the conviction that the same medium or element which is concerned in the setting up of the electro motive tension between metals in contact is present in the streams of Roentgen. It might have been proper to say; in the spirit of more modern views on contact electricity, that these streams are formed by ether, but I have preferred to use the term “primary matter,” for, although the expression “ether” conveys a perfectly definite idea to the scientific mind, there exists, nevertheless, much vagueness as to the structure of this medium. The matter projected is not revealed by spectral analysis, and it does not seem to produce any appreciable mechanical nor even heating effects, nor is it deflected by a magnet, all of which facts tend to show that it can not be composed of molecules of any known substance. The streams exercise a powerful action upon a photographic plate or fluorescent screen, but I look upon these results as obvious consequences of the energetic impact.
Of the various more or less plausible views in regard to the formation of these streams outside of the vessel, the simplest, to my mind, is to assume an actual projection through the walls of the bulb of the disintegrated cathodic matter. Granted that there are particles sufficiently small within the bulb, then all velocities, up to many thousands of kilometers per second, are not only possible, but also probable; and, even if the particles would not be further disintegrated by the impact against the wall or other comparatively opaque body within the bulb, they surely would penetrate through great thicknesses of most substances. My experiments in this direction have shown that all the disintegration is practically accomplished in the first impact against the more or less impenetrable obstacle within the bulb, the second impact having seemingly little effect, as might be inferred from well established mechanical principles. I have also found that the place of first and most energetic impact, be it the anode, cathode or wall of the vessel, is invariably the principal source of the rays or streams. Again, quite in accordance with mechanical principles, the penetrative power of the streams is the greater the more complete the disintegration. Thus, for instance, rays which have traversed thick opaque objects, and are presumably further disintegrated, pass more freely through dense substances. An observation to this effect has likewise been made by Professor Wright, who was the first to publish definite results in the United States. I find that bulbs with thick walls give rays of greater penetrating power. It should be, of course, understood that I do not mean by this a greater outward effect. It is principally the above fact which makes it appear more probable that the matter projected is not a homogeneous stream, but consists of particles of varied magnitude moving with different velocities, for, were the former the case, the penetrative power would depend chiefly on the velocity. In the practical use of the Roentgen streams it would, therefore, seem very important to find a method of filtering and rendering them homogeneous, for only by such a method can we hope to obtain exact results in their investigation. Streams of perfectly uniform velocity and character, if produced, would no doubt be more suitable for the purposes of research.
Since the disintegration of the electrodes, especially if they are of aluminum, is so slow that no appreciable diminution of the weight results even after long use; it follows that the matter conveyed by the Roentgen streams is so minute as to escape detection. Some bulbs, which I have used for a number of months, showed that the bombarded spot of the glass was entirely permeated with particles of the aluminum electrode, but it would probably require years of constant use to accumulate any appreciable amount of matter outside. Referring to a tube with an electrode of aluminum, it is a noteworthy fact that, if properly managed, it does not impair in quality, but, on the contrary, seems to improve; whereas when a platinum electrode is used, the life of the bulb is very short, owing to the conductor being deposited on the walls, which deposit, as I have explained on another occasion, renders difficult the passage of the discharge. Namely, as soon as some of the projected particles strike the conducting layer, they impart a similar electrification to the latter, and a repulsion is exerted upon the particles following. The result is an apparent increase in the resistance of the tube. The above defect of the platinum electrode, despite of its effectiveness, must, in my opinion, lead to its abandonment.
It has been suggested that the Roentgen rays may be due simply to a propagation of electro-static stress; but, on this assumption, it is difficult to conceive how rays could be produced in instances when the glass wall is at a high temperature and consequently conducting, or when the impact plate or inclosure is of metal and connected to the ground. Stokes has recently considered the possibility that the impact of the cathodic stream on one side of a partition might give rise to a molecular motion on the other side without necessarily there being a transit through the partition. According to this view, which I have likewise considered some time ago, it would appear that the material streams might start on the outer side of the wall of the tube, in which case only the air would be responsible for the effects, and the futility of a spectral analysis test would be in a certain measure accounted for. But is it not more probable to assume an actual passage and shattering of matter as all evidences point in this direction? Assuming that, as Professor Stokes now thinks it probable, the disturbance is non-periodic and still capable of producing effects characteristic of transverse vibrations of extremely high frequency, it seems to me a serious question whether, the old Newtonian views on light should not be reconsidered rather than the conclusion drawn that the novel manifestations observed by Roentgen are due to transverse vibrations, when there is no experimental evidence to this effect, nor even a satisfactory explanation found how the cathodic impact might give rise to waves of a higher frequency than those of light.
Being, as I am, firmly convinced of the existence of material streams, I look upon the unsuccess of the attempts of demonstrating an actual transit of matter as being due to either the minuteness of the amount or else to the state of the matter, but rather, to the former cause, as all peculiarities of the streams point in this direction. In my opinion, no experimenter need be deterred from carrying on an investigation of the Roentgen rays for fear of poisonous or generally deleterious action, for it seems reasonable to conclude that it would take centuries to accumulate enough of such matter as to interfere seriously with the process of life of a person. But I look confidently to the demonstration of actions of a purely qualitative nature. For instance, despite of the danger of such an assertion by encouragement which might be given to quacks I would say that I expect with the utmost confidence the demonstration of a germicidal action. In addition to the physiological effects, to which I have early drawn attention, I have more recently observed with powerful tubes that a sensation of pain is produced in the forehead above the eyes just as soon as the current is turned on. This sensation is very similar to that one frequently experiences when stepping from a dark room into the glare of bright sunlight, or when walking for some time over fields of fresh-fallen snow.
As to the hurtful actions on the skin, which have been variously reported, I note that they are misinterpreted. These effects have been known to me for some time, but I have been unable, on account of pressing matters, to dwell on the subject. They are not due to the Roentgen rays, but merely to the ozone generated in contact with the skin. Nitrous acid may also be responsible, to a small extent. The ozone, when abundantly produced, attacks the skin and many organic substances most energetically, the action being no doubt heightened by the heat and moisture of the skin. After exposing the hand, for instance, for some time, the skin loses its elasticity, which causes a tension and pain, and subsequently an inflammation and blistering. This occurs mostly only at short range, but may be produced by a single terminal bulb, or generally by a very highly exhausted bulb, in which the terminals act independently, at greater distance. Owing to this, I have always taken the precaution, when getting impressions with the rays, to guard the person by a screen made of aluminum wires which is connected to the ground, preferably through a condenser. The radical means, however, of preventing such actions is to make impossible the access of the air to the skin while exposing, as, for instance, by immersing in oil. As this would be inconvenient in most cases, a metallic screen should be resorted to. The action of the ozone on some substances, when placed near the bulb in such a way that the gas is generated on their surfaces, is so powerful that the substances are practically destroyed in a few minutes. When a wire heavily insulated with rubber is connected to the terminal of a high-frequency coil, sometimes an exposure of barely a minute is sufficient to completely wreck the rubber insulation. There are certain commercial insulating compounds which are even more quickly destroyed, but which I will not enumerate because of a possible disadvantage to the manufacturers. Gutta-percha, beeswax and paraffine stand the attack very well, and such wires should be used with high-frequency coils. This powerful action of the ozone was observed by me first about two years ago, when performing an experiment which was shown to many persons in my laboratory. The experiment consisted of charging a person, standing on an insulated stand, with a potential approximating one and one-half million volts, which was alternated several hundred thousand times a second. Under such conditions luminous streams break out on all parts of the body, especially abundantly on the feet, hands, hair, nose and ears. I subjected myself a number of times to this experiment, which seemed to offer no other danger except the possible rupture of a blood vessel, if the skin was very dry and non-conducting. I then noted on myself and others after effects resembling much those attributed to the Roentgen rays. With currents produced by perfected electrical oscillators, such as were described in the Electrical Review, September 30, 1896, the production of the ozone is so abundant that it is sufficient to merely turn on the current for a few seconds and ozonize strongly the atmosphere of a large hall. These currents are also capable of bringing about chemical combinations, of which the chief is that of the nitrogen with the oxygen of the atmosphere, and an immense possibility, which I have been following up for a long time, is opened up; namely, the combination of the nitrogen of the atmosphere on an industrial scale by practically no other means than mechanical power. If merely fertilizers of the soil would be manufactured in this manner, the benefits to humanity derived therefrom would be incalculable. From the above named action of the ozone, it follows that the experimenter should use the indicated precaution, for while ozone in small quantities is a most beneficial disinfectant, when generated in large quantities it is not free of danger.
It is an unpleasant duty to say on this occasion a few words on the subject of “making the blind see” by means of the Roentgen rays. This sensational topic has been given a wide circulation in the journals. Is it not cruel to raise such hopes when there is so little ground for it? For, first of all, the rays are not demonstrated to be transverse vibrations. If they were, we would have to find means for refracting them to make possible the projection of a sufficiently small image upon the retina. As it is, only a shadow of a very small object can be projected. What possible good can result from the application of these rays to such purposes? The shape of the small object might eventually be recognized by impressing the retina, but the sense of touch is more than sufficient to convey such impressions. Luminous sensations are well known to be excited in two ways; namely, by mechanical shock and electrical transmission. Both of these, I think, are present in the Roentgen streams, and hence such an effect on the optic nerve might be expected. I may say, however, that I can not confirm some of the experiments reported. For instance, when a hand is put before the closed eyes it is easy to distinguish the shadow, much the same as before the light of a candle; but when the tube is closed, and all light from the same excluded, I fail to get such an impression. The latter is, therefore, chiefly due to ordinary light, or else my tubes act differently from those experimented with by others. It may be proper to recall here that in ordinary bright sunlight, especially in the southern climates, it is easy to distinguish the shadow of objects, and even their rough outlines, with the eyes shut.
Proceeding on the assumption that we have in reality to deal with material streams, it is important to inquire which are the best conditions to be maintained when taking impressions with the sensitive screen or plate. First, the experimenter will easily observe that there are two causes which, with a given bulb and coil, tend to increase the intensity of the impressions. One of these may be said to lie in the bulb, the other in the coil. The latter, being most generally made of many turns of fine wire, is very sensitive to changes in the capacity of bodies attached to its terminals. The capacity of these bodies, therefore, in such a coil largely determines the difference of potential. At a certain degree of exhaustion this capacity assumes such a value that the pressure rises to a maximum, this tending to give the highest velocity to the cathodic stream, and, consequently, to give rise to the most intense rays. But at that degree of exhaustion it may happen, and usually does happen, that the cathodic streams are not most abundant. To produce the best result it is necessary that both of these causes should be made to cooperate by a careful proportioning of the dimensions of the bulb, which, in practice, is very difficult, inasmuch as the experimenter has to avail himself of commercial bulbs which may or may not be best suitable for his coil. This simple consideration shows the great advantage of the use of a coil which contains no fine wire and is capable of giving a heavy current through the secondary far in excess of what even the largest bulb requires.
Assuming the physician has learned how to manipulate his apparatus to best advantage, he will next notice that, to secure the clearest definition, he will have to maintain a certain pressure on the terminals of the tube, dependent chiefly on the distance and degree of opacity of the object investigated. It goes without saying that the definition is the better the smaller the spot from which the rays are emanating, but this is true only when impressions are taken at very small distances. When the distances are large, it is a disadvantage to use a too small radiating surface, as then the density is diminished to such a degree that the action is too weak. Discarding this consideration, it is clear that, if the rays are intense, the more opaque portions of the body are likewise penetrated and much detail is lost, whereas, when the rays are less intense, the impression might be altogether too weak to bring out sufficient detail.
To illustrate in a popular manner the best way to proceed, I shall avail myself of a simple illustration. Suppose that there would be imbedded between two panels of cloth a foreign object, such as a coin, and it is desired to locate it. We may accomplish this by placing behind the cloth a cardboard, for instance, and then firing from a certain distance a load of fine shot through the cloth in, the region where the coin is supposed to be located. The shot will penetrate the cloth on all points except on the place where the coin is located, and on the cardboard behind, this place will be plainly indicated by the absence of the marks. Exactly in this way we proceed in applying the Roentgen rays to the location of such a body. Roentgen gave us a gun to fire — a wonderful gun, indeed, projecting missiles of a thousandfold greater penetrative power than that of a cannon ball, and carrying them probably to distances of many miles, with velocities not producible in any other way we know of. These missiles are so small that we may fire them through our tissues for days, weeks, months and years, apparently, without any hurtful consequence. Instead of the cardboard to indicate the path of the missiles, he gave us what is properly called a Roentgen screen, which becomes luminous on all places where it is hit by the missiles. Where the latter are prevented from hitting the screen by the intervention of the opaque body, the screen does not glow and we observe the shadow of the object. It is simple enough to project the shadow of an object in this way but when it is required to show the finer detail of the structure of the object, the difficulty begins. It will at once appear that, to produce such a result to best advantage, two conditions will have to be more or less realized. Firstly, the screen should be composed of such material that it is capable of becoming luminous by the faintest impact; and, secondly, the missiles should all be of uniform size, and should move with uniform velocity. Neither of these two conditions has so far been realized in practice, for all the bodies we know require a violent impact to become luminous, and no way has been found as yet to produce a uniformity in velocity and magnitude of the supposed projectiles. But a little thought leads immediately to the conclusion that there will be a certain velocity of the missiles which will give, under all conditions. the best definition. This velocity is easily ascertained by trial. Evidently the definition will be best when the bullets which pass through the densest parts of the body strike the screen so feebly as to not make it light up, whereas, those passing through portions of slightly smaller density hit it sufficiently strong as to make it light up feebly. The more sensitive the screen to impact, that is, the weaker an impact is required to make the screen light up, the more detail will be revealed. It therefore follows that, in the application of the Roentgen rays, not the body which fluoresces strongest, but the one which is most sensitive, is best suited for finer work.
The above considerations have led me to adopt the following procedure, which, in practice, has proved very successful. The Roentgen screen is first applied to the body to be investigated, the pressure at the terminals of the tube being very much reduced. The pressure is then slowly and gradually raised. It will be presently observed than at a certain pressure, the shadow of the object examined is clearest. But as the vacuum is increasing, the pressure generally rises, and the image gets blurred in spite of the screen getting much brighter. Just as soon as the clearness is slightly diminished, the experimenter should for a few moments reverse the current, lowering a little the vacuum in this manner. The current being again given the direction it had at first, namely, that which causes a slow and steady increase of the vacuum, the shadow gets again clear, and by such easy manipulation the best result may be secured. An additional advantage, however, is gained, because the frequent reversals produce a brighter phosphorescence of the screen. When taking a photograph, the bulb should be watched through the screen and the switches manipulated in the above manner.
To give a practical example of the effectiveness of this procedure, I need only mention one of the instances which have come to my notice. A few months ago I investigated the case of Mr. Cornelius Mack, of Watertown, Mass. Mr. Mack, while performing his duties many years ago, was struck by a bullet which lodged somewhere in the chest and could not be located. I apply the screen vainly a number of times for although the streams penetrated the body with such ease as to make the screen behind appear bluish white, and reveal all the bones of the body, I could not observe the missile. I then resorted to the above indicated means, and immediately, and easily the exact location of the projectile, between the shoulder blade and one of the ribs, was ascertained and the bullet successfully extracted.