Letter: Tesla's Latest Roentgen Ray Investigations

Wednesday, April 22, 1896
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/*Pm 2% 1396 ELECTRICAL REVIEW 207 TESLA’S LATEST ROENTGEN RAY INVESTIGATIONS. To 'ras Emrou or Emr,c'rs.1cu. Rmvuzws Further investigations concerning the behavior of the various metals in regard to rellection of these radiations have given additional support to the opinion which I have beforeexpressed; namely, that Volta’s electric contact series in air is identical with that which is obtained when arranging the metalsaccording to their powers of reflection, the most electro-positive metal being the best retlector. Con- fining myself to the metals easily ox- perimented upon, ' this series is magnesium, lead, tin, iron, copper, silver, gold andfplatinum. The last- named metal should be found to be the poorest, and sodium one of the best, reflectors. This relation is ren- dered still more interesting and sug- gestive when we consider that this series is approximately the same which is -obtained when arranging the metals according to their energies of combination with oxygen,as calcu- lated from their chemical equivalents. Should the above relation be con- firmed by other physicists, we shall be justified to draw the following con- clusions : First, the highly exhausted bulb emits malerial streams which, impinging on a metallic surface, are rellected; second, these streams are formed of matter in some primary or elementary condition 3 third, these material streams are probably the same agent which is the cause of the electro~motive tension between metals iu close proximity or actual contact, and they may possibly, to some extent, determine the energy of combination of the metals with oxygengfourth, every metal or conductor is more or less a source of such streamsgjifth, these streams or radiations must be produced by some radiations which exist in the medium; and sin'/71, streams resembling the cathodic must be emitted by the sun and probably also by other sources of radiant energy, such as an arc light or Bunsen burner. The first of these conclusions, as- suming the above-cited fact to be cor- rect. is evident aud uncontrovertible. No theory of vibration of any kind would account for this singular rela- tion between the powers of reflection and electric properties of the metals. Streams of projected matter coming in actual contact with the redecting metal surface afford the only plausi- ble explanation. The second conclusion is likewise obvious, since no didercnce whatever is observed by employing various qualities of glass for the bulb, elec- trodes of different metals and any kind' of residual gases. Evidently, whatever the mutter constituting the streams may be, it must undergo a change in the process of expulsion, or, generally speaking, projection- since the views in this regard still differ-in such ri nay as to lose en- tirely the Cll1ll`{lCi@l‘lShlCS which it possessed when forming the electrode, or wall of the bulb, or the gaseous contents of the latter. The existence ofthe above relation between the retlcting and contact series forces us likewise to the third conclusion, because a mere coinci- dence of that kind is, to suy the least, extremely improbable. Be- sides, the fact may be cited that there is always ;i.iliEe1'ence of potential set up between two metal plates at some distance and in the path of the rays issuing from an exhausted bulb. Now, since there exists an electric pressure or dilference of potential between two metals in close prox- imity or contact, \ve must, when cou- sidering all the foregoing. come to the fourth conclusion, namely, that the metals emit similar streams, and I therefore anticipate that, if a. sensi- tive tilm be placed betweeuttvo plates, say, of magnesium and copper, at true Roentgen shadow picture would bc obtained after a very long exposure in the dark. Or, in general, such picture could be .secured . whenever the plate is placed near a metallic or conducting body, leaving for' the present the insulators out of consid- eration. Sodium, one of the first of the electric contact series, but not yet experimented upon, should give out more of such streams than even magnesium. Obviously, such streams could not be forever emitted, unless there is a continuous supply of radiation from the medium in some other form; _or possibly the streams which the bodies themselves emit are merely reflected streams coming from other sources. But since all investigation has strengthened the opinion advanced by Roeutgen that tor the production of these radiations some impact is required, the former of the two pos- sibilities is the more probable one, and we must ussume that the radia- tions existing in the medium and giving rise to those here considered partake something of the nature of cathodic streams. But if such streams exist all around us in the ambient medium, the ques- tion arises, whence do they come? The only answer is : From the sun. I infer, therefore, that the sun and other sources of radiant energy must, in a less degree, emit radiations or streams of matter similar to those thrown elf by an electrode in rt highly exhausted iuclosure. This seems to be, at this moment, still it point of controversy. According to my present convictionsa Roentgeu shadow Dwi- urs should, with very long exposures, be obtained from all sources oE_rndiant energy, provided the radiations are permitted Erst to impinge upon 2 metal or other body. The preceding considerations tend to show that the lumps of matter composinga cathodio stream in the bulb are broken up into incompar- ably smaller particles by lmpfwi- against the wall of the latter,uncl, owing to this, are enabled to p iss into the air. All evidence which I have so far obtained points rather to_this than to the throwing od of particles of the wall itself under the violent impact ofthe cathodic stream. Ac- cording to my convictions, then, the difference between Lenurdatnil llocut- geu rays, if there be any.lies solely in this, that the particles composing llw latter are inconiparably smaller and possess a higher velocity. To these two qualibcations I chiefly attribute the non-delleotibility by o magnet which I believe will be disprovcd in the end. Both kindsofrays,l1oweve1‘, adect the sensitive plate and fluores- cent screen. only the rays discovered byvltoeutgcn are much more effective. We know now that these rays are pro- duced under certain exceptional con- diticnsin a bulb. the vacuum being extremely high. and that the rouge of greatest activity is rather small. y I have endeavored to find wliether the retl-acted rays possess certain dis- tinctive features, and I have taken pictures of various objects with this purpose in view, but no marked difference was noted in any case. I therefore conclude that the matter composing thc Roentgen rays does not surfer further degrutlution by impact against bodies. One ofthe most important tasks for the experi- menter reinnins still to determine what becomes of the energy of these rays. In ri number of experiments with rays reflected from and trans- mitted through u, conducting or insu- lating pl1te,I fouml that onlya small part of the rays could be accounted for. For instance, through a zinc plate, one-sixteenth ol' an inch thick, un»ler an incident ungle of -15 degrees, about two and o:ie~half per cent were reflected and about three per cent transmitted through the plate, hence over 94 per cent of the total radiation remain to be accounted for. All the tests which I have been ablc to make have confirmed Roentgen’s statement that these rays are incapable of rais- ing the temperature of a borly. To trace this lost energy and account for it in a plausible way will be equiva- lent to mal-.ing a new discovery. Since it is now demonstrated that all bodies reflect more or less, the diffusion through the air is easily accounted for. Observing the tend- ency to scatter thrbugh the air, Ihave been led to increase the ediciency of reliectors by providing not one, but separated successive layers for reilcction. by making the reflector of Lhiu sheets of metal, mica or other substances. The efficiency of mica as a reflector I attribute chiefly to the fact that it is composed of many superimposed layers which reilect in- dividually. These many successive reflections ure, in my opinion, also the cause of the scattering through thc air. In my communication to you of April l, I have for the ‘first time stated that these rays are composed of matter in a, “ primary ” or element- ary condition or state. I have chosen this mode of expression in order to avoid the use of the word "ether," which is usually understood in the sense of the Maxwellian interpreta- tion, which would not be in accord with my present U‘,\llYlCl.lO]'lS in regard to thc nature ol’ the rndiatinns. An observation which might be of some interest is the following: A few years ago I described on one occasion a phenomenon observed in highly exhausted bulbs. It is a brush or stream issuing from a. single elec- trode Linder certain conditions, which rotates very rapidly in consequence of the action of the earth`s magnet- ism. Now I have recently observed this same phenomenon in sew-ral bulbs which were capable of im- pressing the sensitive film and fluo- rescent screen verysrrongly. As the brush is rapidly xtvirling around I have conjectured that perhaps also the _Lenard and Roentgen streams are rotating under the action of the cartli’s nr-iguetism,and I amendezivor- ing to obtain an eviilence. of such motion liy studying the action of a bulb in various positions with respect to the magnetic :isis of the earth. In so far us the vibrational charac- ter of the rays is conceruedgl still liild that the vibration is merely that which is conditioned by the appara- tus employed. With the ordinary induction coil we have almost exclu- sively to deal with il very low vibra- tion iniprcssed bj: the comniutziting device or brul;e. With the disruptive coil we usually l}1l\'€‘ a very strong sxipcriinpnsefl \'il»mtion in addition one, and it is to the iuudaineuml easy to trace sonieiiuics as much as the fuudeuiental the fourth owtaue of vibration. But I run rot reconcile idea of vibrations even exceeding think that ull be as well pro- nijsself with the approximating or those of light, and these 9-Heats could duced with ri steady electrical press- ure as from a. battery, with the exclusion of all vibration which may occur, even in such instance, as has been pointed out by De La Rive. In my experiments 1- have tried to ascer- tain whether a greater difference between the shadows of the bones and flesh could be obtained by employ- ing currents of extremely high fre- quency, but I have been unable to discover any such effect which would be dependent on the frequency of the currents, although the latter were varied between as wide limits as was possible. But it is a rule that the more intense the action the sharper the shadows obtained, provided that the distance isnot too small. It is furthermore of the greatest impor- tance for the clearness ofthe shadows that the rays should be passed through some tubular reflector, which renders them sensibly parallel. In order then to bring out as much detail as p>ssible on a sensitive plate, we have to proceed in precisely the same way as if we had to deal with flying bullets hitting against a wall composed of parts of different density with the problem before us of produc- ing as large as possible a difference in the trajectories of the bullets which pass through the various parts of the wall. Manifestly, this difference will be the greater the greater the velocity of the bullets; hence, in order 'to bring out detail, very strong radia- tions are required. Proceeding on this theory Ihaye employed exception- ally thick Elms and developed very slowly,and in this way clearer pictures have been obtained, The importance of slow development has been lirst pointed out by Professor Wright, of Yule. Of course, if Professor Henry’s suggestion of the use of a fluorescent body in contact with the sensitive Hlm is 'mhde use of, the process ,is ,reduced to an ordinary quick photo- graphic procedure, and the above consideration does not apply. lt being desirable to produce as powerful a radiation as possible, I have continued to devote my atten- tion to this problem and have been quite successful. First of all, there existed limitations in the vacuum tube which did not permit the apply- ing of as high a potential as Idesired; namely, when a certain high degree of exhaustion was reached a spark would form behind the electrode, which would prevent straining the tube much higher. This inconven- ience 1 have overcome entirely by making the wire leading to the elec- trode very long and passingit through a narrow channel._so that the heat from the electrode could not` cause the formation of such sparks. An- other limitation was imposed by streamers which would break out at the end of the tube when the potential was excessive. This latter inconvenience I have overcome either by the use of a cold blast of air along the tube. as I have mentioned before, or else by immersion of the tube in oil. The oil, as it is now well known. is a means of rendering impossible the formation of streamers bythe exclusion of all air. The use of the oil in connection with the pro- duction of these radiations has been early advocated in this country by Pro- fessor Trowbridge. Originally l em- ployed a wooden box made thor- oughly light with wax and filled with oil or other liquid, in which the tube was immersed. Observing certain specido actions, I modified and im- proved the apparatus. and in my later investigations I have employed an urraingement as shown in the an- nexed cut. A bulb b, of the kind described before, with a leading-in wire and neck much longer than here shown, was inserted into a large and (concluded fm page 2l1,)

April 22, 1896 TE5LA'5 LATEST ROENTGEN RAY INVESTIGATIONS. (Cancluded from page :om thick glass tube t. The tube was closed in front by a diaphragm fl of pergament, and by a rubber plug P in the back. The plug was provided with two holes, into the lower one of which u glass tube f,, reaching to very nearly the end of the bulb, was inserted. Oil of some kind was made to flow through rubber tubes r rfrom a large reservoir R, placed on an ad- justable support S, to the lower reser- voir R,, the path of the oil being clearly observable from the drawing. . m J g o z; -. ll‘ l By adjusting the difference of the level between the two reservoirs it was easy to maintain a permanent condition of working. The outer glass tpbe t served in part as a re- flector, while at the same time it per- mitted the observation of the bulb I: during the actionl The plug P, in which the conductor o was tightly sealed. was- so arranged that it could be shifted in and out of the tube t, so as to vary the thickness of the oil traversed ,by the rays. I have obtained some results with this apparatus which gl'early show'the advantage of such disposition. For instance, at a distance of 45 feet from the end of the bulb my assistants and myselfcould observe clearly the fingers of the hand through a screen of tungstate of calcium, the rays trav- ersing about two and one-half inches of oil and the diaphragm d. It is practicable with such apparatus to make photographs of small objects at a distance of 40 feet, with onlya few_ minutes exposure, by the help of Professor Hcnry’s method. But, even without the use of a fluorescent powder, short exposures are prac- ticable, so that I think the use of the above method is not essential for quick procedure. I rather believe that in the practical development of this principle, if it shall be necessary, Professor Salvioni’s suggesti-~n of a fluorescent emulsion, combined with a film, will have to be adopted. This is bound to give better results than an independent fluorescent screen, and will very much simplify the process. I may say, however, that, since my last communication, con- siderable improvement has been made in the screens. The manufacturers of Edison’s tungstate of calcium are now furnishing screens which give fairly clean pictures. The powder is fine and it is more uniformly distributed. I consider, also, that the employment- of a softer and thicker paper than before is of advantage. It 'is just to remark that the tungstate of calcium has also proved to be an excellent duorescent in the bulb. I tested its qualities for such use immediately and find it so far unexcelled. Whether it will be so for along time remains to be seen. News reaches us that several fluores- cent bodies, better than the cyanides, have been discovered abroad. Another improvement with a view of increasing the sharpness of the ELECTRICAL REVIEW 211 shadows has been proposed to me by Mr. E. R. Hewitt. He assumed that the absence of sharpness of the out~ lines in the shadows on the screen was due to the spread of the fluores- cence from crystal to crystal. He proposed to avoid this by using a thin aluminum plate with many parallel grooves. Acting on this suggestion, I made some experiments with wire gauze, and, furthermore, with screens made of a mixture ofifa fluorescent with a non-fluorescent powder. I found that the general brightness of the screen was diminished, but that with a strong radiation the shadows appeared sharper. This idea might be found capable of usefulapplication. By the use of the above apparatus I; have been enabled to examine much better than before the body by means ofthe fluorescent screen. _Presently the vertebral column can ,be seen quite clearly, even in the lower part of the body. I have also clearly noted the outlines of the hip bones. Look- ing in the region of the heart I have been able to locate it unmistakably. The background appeared ,much brighter, and this difference in the intensity of the shadow and surround- ing has surprised me. The ribs I could now see on a number of occa- sions quite distinctly, as well as the shoulder bones. Of course, therais no didiculty whatever in observing the bones of all limbs. I noted cer- tain peculiar effects which I at- tribute to the oil. For instance, the rays passed through plates of metalover one-eighth ofan inch thick, and in one instance I could see quite clearly the bones of my hand through sheets of copper, iron and brass of a thickness of nearly one-quarter of an inch. Through glass the rays seemed to pass with such freedom that, look- ing through the screen in a direction at right angles to the axis of the tube, the action was most intense, although the rays had to pass through a great thickness of glass and oil. A glass slab nearly one-half of an inch thick,' held in front of the screen, hardly dimmed the Huo- rescence. When holding the screen in front of the tube at a distance of about three feet, the head of an assist- ant, thrust between the screen and the tube, cast but a feeble shadow. It appeared some times as if the bones and the Hesh were equally transparent to the radiations passing through the oil. When very close to the bulb, the screen was illuminated through the body of an assistant so strongly that, when a hand was moved in front, I could clearly note the motion of the hand through the body. In one instance I could even distinguish the bones of the arm. Having observed the extraordinary transparence of the bones in some instances, I at first surmised that the rays might be vibrations of high pitch, and that the oil had in some Way absorbed u part of them. This view, however, became unten- able when I found that ata certain distance from the bulb I obtained a sharp shadow of the bones. This latter observation led me to apply usefully the screen in taking im- pressions on the plate. Namely, in such case it is of advantage to first determine by means of the screen the proper distance at which the object is to be placed before taking the impression. It will be found that often the image is much clearer at a greater distance. In order to avoid any error when observing with the screen, I have surrounded the box with thick metal plates, so as to prevent the duorescence, in conse- quence of the radiations, reaching the screen from the sides. I believe that such an arrangement is absolutely necessary if one wishes to make cor- rect observations. During my study of the behavior of oils and other liquid insulators, which I am still continuing, it has occurred to me to investigate the important elfect discovered by Prof. J. J. Thomson. He announced some timeago that all bodies traversed by Roentgen radiations become con- ductors of electricity. I applied a sensitive resonance test to the inves- tigation of this phenomenon in a manner pointed out in my earlier writings on high frequencycurrents. A secondary, preferably not in very close inductive relation to the primary circuit, was connected to the latter and to the ground, and the vibration through the primary was so adjusted that true resonance took place. As the secondary had a considerable number of turns, very small bodies attached to the free terminal produced gdnnsiderable variations of potential on 'the latter. Placing a tube in a box of wood 'dlled with oil and attaching it to the terminal, I adjusted the vibration through the primary so that resonance took place without the bulb radiating Roentgen rays to an appreciable extent. I then changed the conditions so that the bulb became very active in the production of the rays. The oil should have now, according to`Prof. J. J. Thomsou’s statement, becomejx conductor and a very marked change in the vibration should have occurred. This was found not to be_ the case, so that we must see in the phenomenon dis- covered by J. J. Thomson only a further evidence that we have to deal here with streams of matter which, traversing the bodies, carry away electrical charges. But the bodies do not become conductors in the common acceptance of the term. The method I have followed is so delicate that a mistake is almost an impossibility. Nrxotx Tasnx. New York, April 20.