137. Tesla’s Experiments. Elec. Rev., N.Y., March 11, ’96, page 131, March 18, page 147, April 1, page 171, and April 8, page 183. Kind of Electrical Apparatus for Operating Discharge Tubes for Powerful X-rays. § 106, 109, 114, 131. The experiments performed by Nikola Tesla were particularly noteworthy for the magnitude and intensity of the rays generated by his apparatus, under his skilful manipulation of the adjustments and circuits particularly as to resonance. The remarkable results that he obtained are not surprising when we learn that he employed his well-known system for producing exceedingly enormous potential and unusually high frequency. § 51. The primary electrical generator as he indicated and as apparent from his system referred to in the above section, could be either a direct or alternating current generator, or other form. If the first is employed, of course an interrupter is necessary in order that there may be a current induced in the secondary.

Mr. Oliver B. Shallenberger, (Mem. Amer. Inst. Elec. Eng.) whose laboratory is in Rochester, Pa., gave some important general instructions concerning the Tesla system § 51, that he employed in the production of remarkably clear sciagraphs, in conjunction with the focus tube, § 91, representing the hand at page 68, and showing a rat shown at this § 137. (Elec. World, N.Y., March 17, ’96.) Even the ligaments were clearly shown in the sciagraph of the rat, and some of them are dimly reproduced by the half tone process. As to the apparatus and operation, which are especially important, it may be stated that the current was taken from an alternator, of a frequency of 133 periods per second, and passed through a primary coil of a transformer for increasing the E. M. F. from 100 volts to from 16 to 25 thousand. The secondary current was then passed through Leyden jars and a double cascade of slightly separated brass cylinders, whereby it was changed into an oscillatory current of an extremely high frequency, which was then conducted through the primary of a second induction coil having very few turns of wire, and no iron core and having a ratio of 7 to 1. By this means the E. M. F. was raised to somewhere between 160,000 volts to 250,000, and was used to energize the discharge tube for the generation of X-rays. Caution should be taken, because the current coming from the first transformer, being of large quantity and very high E. M. F. is exceedingly dangerous, but the current of the second secondary has been passed through one’s body without danger, as reported by Mr. Tesla several years ago, and confirmed by Mr. Shallenberger.

138. Phosphorescent Spot Maintained Cool.—In testing the power of the X-rays in connection with the appearance of the phosphorescent spot, Tesla noticed that they were most powerful when the cathode rays caused the glass to appear as if it were in a fluid state. § 61. To prevent actual puncture, he maintained the spot cool by means of jets of cold air. It became possible thereby to use bulbs of thin glass at the location of the generation of the X-rays. § 119. He concluded from certain results that not only was glass a better material for discharge tubes than aluminum, but because, by other tests, he found that thin aluminum cast more shadow with X-rays than thicker glass. There are, of course, many other reasons, based on mechanical construction, why glass is preferable, and also why a tube with an aluminum window is not to be desired. Principally, the latter will soon leak.

139. Expulsion of Material Particles through the Walls of a Discharge Tube.—At quite a low vacuum, and after sealing off the lamp, he attached its terminal to that of the disruptive coil. After a while, the vacuum became enormously higher, as indicated by the following steps: First, a turbid and whitish light existed throughout the bulb. This was the first principal characteristic. Next, the color changed to red, and the electrode became very hot, in that case where powerful apparatus was employed. The precaution should be taken to regulate the E. M. F., to prevent destruction of the electrode. Gradually, the reddish light subsided, and white cathode rays, which had begun, grew dimmer and dimmer until invisible. At the same time, the phosphorescent spot became brighter and brighter and hotter and hotter, while the electrode cooled, until the glass adjacent thereto was uncomfortably cold to the touch. At this stage, the required degree of exhaustion was reached, and yet without any kind of a pump. He was enabled to hasten the process by alternate heating and cooling, and by the use of a small electrode. This whole phenomenon was exhibited with external electrodes as well. He acknowledged that instead of the disruptive coil, a static machine could be used, or, in fact, any generator or combination of devices adapted to produce a very high E. M. F.

The reduction of temperature of the electrode he attributed to its volatilization. Without actually testing the rays with a fluorescent screen or photographic plate, he could always know their presence by the relative temperatures of the phosphorescent spot and the electrode, for when the latter was at a low temperature and the former at a high temperature, X-rays were sure to be strong.

From the fact that the vacuum became higher and higher by the means stated, he was very much inclined to believe that there was an expulsion of material particles through the walls of the bulb. When these particles which were passing with very great velocities struck the sensitive photographic plate they should produce chemical action. He referred to the great velocity of projected particles within a discharge tube, pages 46 and 47, and to Lord Kelvin’s estimate upon the same, and reasoned that with very high potentials, the speed might be 100 km per second. The phenomenon indicated, he said, that the particles were projected through the wall of the tube and he entered into an elaborate discussion on this point. He referred to his own experiment of causing the rays from an electrode in the open air to pass directly through a thick glass plate. § 13. He performed an experiment also of producing a blackening upon a photographic plate apparently by the projected particles, an electrical screen being employed to prevent the formation of sparks. § 35. which as well known will cause chemical action upon the plate. No stronger proof as to the expulsion of material particles could be desired than an operation in which the eyes can see for themselves that such an action must have taken place. Usually he was troubled by the streamers (cathode rays) from the electrode suddenly breaking the glass of the discharge tube. This occurred when the spot struck was at or near the point where the same was sealed from the pump. He arranged a tube in which the streamers did not strike the sealing point, but rather the side of the tube. It was extraordinary that a visible but fine hole was made through the wall of the tube, and especially that no air rushed into the vacuum. On the other hand, the pressure of the air was overcome by something rushing out of the tube through the hole. The glass around the hole was not very hot, although if care were not taken, it would become much hotter, and soften and bulge out, also indicating a pressure within, § 27. greater than the atmospheric pressure. He maintained the punctured tube in this condition for some time and the rarefaction continued to increase. As to the appearances, the streamers were not only visible within the tube, but could be seen passing through the hole, but as the vacuum became higher and higher, the streamers became less and less bright. At a little higher degree of vacuum, the streamers were still visible at the heated spot, but finally disappeared.

This electrical process of evacuating varies in its rapidity according to the thinness of the glass. Here again he noted the application of his theory in that an easier passage was afforded for the ions. § 47. A few minutes of operation produced through thin glass, a vacuum from very low to very high, whereas, to obtain the same vacuum through much thicker glass over 1/2 hour was necessary. Again with a thick electrode the time required was much greater. The small hole was not always visible and it was thought that the material went through the pores. The result obtained by the following experiment tends to uphold Mr. Tesla’s emission theory.

139a. Lafay’s Experiment. Giving to X-rays the Property of Being Deflected by a Magnet by Passing Them Through a Charged Silver Leaf. Comptes Rendus, March 23, ’96 and April 7, 13, 27, and L’Ind. Elec., April and May ’96. From trans. by Louis M. Pignolet. He placed at about .5 cm. below a discharge tube, a lead screen pierced by a slit 2 mm. wide; and 0.04 m. lower, a second lead screen having a slit 5 mm. wide completely covered by an extremely thin leaf of silver. Opposite the silver leaf and exactly in the axis of the slit, was fixed a platinum wire 1.5 mm. diameter. Thus, the rays which passed successively through the two slits projected a shadow of the wire on a photographic plate below.

When the silver leaf was connected to the negative pole of the induction coil that excited the tube, the rays, which had become electrified (§ 61b, p. 47) bypassing through the leaf, were deflected by a magnetic field of about 400 L. G. S. units, whose lines of force were parallel to the slit. The direction of the deflection was determined by the same law as that of the deflection by a magnetic field of the cathode rays in the interior of a discharge tube. § 59. When the silver leaf was not connected to the coil, no deflection was produced. § 79.

To double the apparent deflection, one part of the slit was covered by a lead plate during the first half of the experiment. The lead plate was removed and placed over the other part of the slit, and the direction of the magnetic field reversed during the last half of the experiment. Thus the distance on the sciagraph between the two parts of the wire, was double the deflection produced by the magnetic field.

The deflection was in the same direction when the silver leaf was connected to the negative pole of a static electric machine, but was in the opposite direction when the leaf was connected to the positive pole of the machine. The test was criticised in the scientific press, and, therefore, in order to be certain that the deflections observed were not due to the combined effects of the electro-magnet which produced the magnetic field and the electric field of the charged silver leaf, the experiments were modified. To remove this uncertainty, the electrified rays were caused to enter a grounded Faraday cylinder (see figure at E. F. G. H., p. 47), through a small opening, before passing between the poles of the electro-magnet. The deviations which were recorded on a photographic plate in the box were the same as before.

Additional experiments showed that the deflections by the magnetic field took place as well when the rays were electrified, after their passage through another magnetic field, as before. Lafay continued the experiments in great detail and by many control tests, and he took accurate measurements and followed the suggestions of others. It would be well for those who have facilities to repeat these most interesting and important researches, to determine for themselves some satisfaction.

It is of interest to note that an American, Paul A. N. Winand, (Mem. Amer. Inst. Elect. Engs.), in the absence of facilities for experimenting, proposed (Elect. World, N.Y., June 6, ’96) to interpose a hollow sphere, which had high potential, in the path of X-rays, and to learn in what manner, if any, the rays are influenced. He argued that it would seem natural that, inasmuch as the rays produce a discharge, there should be a reaction of the charged surface upon the rays.

It is evident that if any one repeats these experiments, expert manipulation is required.

139b. Gouy’s Experiments. The Penetration of Gases into the Glass Walls of Discharge Tubes. Comptes Rendus, March 30, ’96. From trans. by Louis M. Pignolet. From observations with slightly different glass from four tubes, it seemed that the cathode rays cause the gases in the tubes to penetrate the glass where they remain occluded until the glass is nearly softened (after cutting off the current), by heat, whereby they are set at liberty as minute bubbles visible by the microscope, which finally partly combine and become visible to the naked eye.

Under the same conditions, tubes which have been used for a long time exhibit numerous wrinkles, indicating a superficial modification of the glass. These may exist with or without the bubbles.

140. Discharge Tube Surrounded by a Violet Halo. By means of enormous potential and high frequency, the tube was surrounded, Tesla stated, by violet luminosity or halo. § 6. and 74. From the fact that Lenard obtained a similar appearance in front of the aluminum window, it might be reasonable to conclude that there is some close relationship between the two phenomena.

As an illustration of halo by light, may be mentioned the well known appearance so often occurring in the atmosphere concentrically with the moon, and sometimes surrounding the sun. Under favorable circumstances, (in a mist or dust in the air), a halo, at some distance from a flame or other light is faintly visible. It has generally been assumed that the reason of a halo by light is based upon the laws of reflection, or refraction or both, the bending of the rays taking place, through, or upon the surface of the particles of moisture. Others have held that particles of ice in the upper atmosphere, are the reflectors or refractors, or both. More puzzling has been the attempt to explain the novel appearance reproduced fairly well in the cut, page 140. It is here represented in print for the first time, but the photograph from which it was taken, was at various times, shown to different physicists, some of whom attributed the beautiful effect to the property of interference of light, and naming Newton’s rings as an analogous production. Prof. Anthony in an interview expressed himself as well satisfied that interference could not occur under the circumstances named. He recognized that there was a curved surface of glass which might be considered as made up of an infinite number of layers. The author introduces the matter for the purpose of consideration by others, and especially because it is so intimately connected with the subject of the vacuum tube and electricity. The details must be understood for the purpose of proper appreciation. Mr. William J. Hammer, of New York, had a photograph taken of the large Concert Hall at the Crystal Palace, Sydenham, Eng., by the light of the Edison incandescent lamps with which the Hall was illuminated. This photograph was made in 1882 during the International Electrical Exhibition held at the Crystal Palace. The picture shows a small section of the whole photograph and represents (although probably no one would judge so by looking at the picture) a festoon of pear-shaped incandescent electric lamps, each one hanging downward, and separated from its neighbor by between three and four feet. They were so far away from the camera that a picture of the lamps unlighted, would have represented them as mere specks. The bright circles with the bright central crosses in the centre of the dark spaces were, therefore, fully one foot in diameter, while the lamp bulbs themselves were only about two or three inches thick, as usual. Why then should there be the halos? Why should the crosses appear? And why should the black area be so large? If the electricity and vacuum have nothing to do with it, why should not the halos appear when photographs are taken of flames and other sources of light in the absence of mist and dust? In order to answer questions which will perhaps be proposed, let it be stated that there was no visible dust nor moisture in the room, the photograph being taken early in the evening and at a time when the Hall was not in use. The halos were not apparent except when reproduced by a photograph. The lamps had the usual carbon filaments hanging so that the several filaments were in different planes, and they were of 16 candle power and were connected in parallel circuit, the average E. M. F. being about 110 volts. The lamps were fed by the Edison direct current dynamos. The festoon shown, is one of a dozen or more which were suspended between the columns rising from the gallery and supporting the roof and were hung about forty feet from the floor. The hall was further illuminated by a huge electrolier pendant from the centre of the ceiling. These details were obtained from Mr. Hammer, who planned the installation.

141. AnÆsthetic Properties of X-rays.—Tesla reported that he and his assistants tested the action of the rays upon the human system, and found that upon continued impact and penetration of the head by very powerful radiations, strange effects were noticed. He was sure that from this cause a tendency to sleep occurred (§ 84, at end), and the faculties were benumbed. He said that time seemed to pass quickly. The general effect was of a soothing nature, and the top of the head seemed to feel warm under the influence of the rays. Incidentally, he noticed, as he stated, “When working with highly strained bulbs, I frequently experienced a sudden and sometimes even painful shock in the eye. Such shocks may occur so often that the eye gets inflamed, and one cannot be considered cautious if he abstains from watching the bulb too closely.”

The author calls to mind the reports in the daily press that Edison also noticed that the eyes were in some way sensitive to the rays. The eye, it was reported, became fatigued at the time, and yet later, objects could be more easily distinguished.

In this connection, it should be remembered that there are not only cathode rays, X-rays, etc., but the electric force that Lenard spoke of in the neighborhood of the discharge tube, and in testing the effects upon the eyes, of course, the precaution should be taken to determine whether cathode rays, X-rays or the electric sparks are answerable for the peculiar effects. Roentgen reasoned, § 84, that the eyes were not sensitive, but the rays, in his case, were not strong enough to travel 40 to 60 feet, as in Tesla’s experiments, but only 2 m. (about 7 ft.).

142. Sciagraphs of Hair, Fur, Heart, Etc., by X-rays.—Tesla was probably the first to be at all successful in the representation in sciagraphs of such objects as hair and cloth and similar easily permeable objects. In the case of a rabbit, not only was the skeleton visible, but also the fur. Sciagraphs of birds exhibited the feathers fairly distinctly. The picture of a foot in a shoe not only represented the bones of the foot, and nails of the shoe, but every fold of the leather, trowsers, stockings, etc. His opinion as to the useful application of the rays was that any metal object, or bony or chalky deposit could be “infallibly detected in any part of the body.” In obtaining a sciagraph of a skull, vertebral column, and arm, even the shadows of the hair were clearly apparent. It was during such an experiment that the anÆsthetic qualities were noticed. The author saw several of the above named sciagraphs. Furthermore, on the screen he believed he detected the pulsations of the heart. Elect. Rev., N.Y., May, 20, ’96.

Although we do not doubt this report concerning what Mr. Tesla saw, yet some scientific men are exceedingly dubious concerning the results obtained by other scientists, unless the same are confirmed by additional witnesses. It will certainly be of interest to such skeptics to have corroboratory evidence. In company with Prof. Anthony, Mr. Wm. J. Hammer and Mr. Price, editor of the Elect. Rev., N.Y., the author visited a laboratory at 31 West 55th street, New York City, for the purpose of beholding the pulsations of the human heart by means of an experiment performed by Mr. H. D. Hawkes, of Tarrytown, N.Y. There was nothing new about his apparatus, the admirable results being due merely to accurately proportioned electrical and mechanical details and skillful manipulation. The Tesla system was not used, but merely a large induction coil and rotary interrupter, and a direct current from the incandescent lamp circuit of 110 volts, all substantially as Roentgen himself employed. The sciascope was provided with the Edison calcic tungstate screen. In order to overcome the sparking between the terminals on the outside of the tube after a few minutes of use, he heated the cathode end by means of a Bunsen burner flame. § 139, near beginning. The utility consisted in the evaporation of condensed moisture upon the cool end of the discharge tube. The temporary heating always prevented, for several minutes, any sparking on the outside. After some preliminary experiments, each person, in turn, pressed the sciascope upon the breast of another, at the location of the heart, while the discharge tube was directly at the back of a young man. The ribs and spinal column were visible, and, projecting from the spine, appeared a semi-circular area, which expanded and contracted. Any one viewing such an operation, and knowing that he is looking at the movements of the heart, cannot but be impressed with weird wonder, and cannot but credit great honor, not only to Roentgen and Lenard, but to all those early workers who have gradually but surely, successfully made discovery after discovery in the department of the science of discharges, finally culminating in the most wonderful discovery of all.

The author remembers seeing in some medical paper that William J. Morton, M.D., of New York, had also witnessed the beating of the heart with the sciascope at an early date. Similar reports are occurring weekly.

§ 142a. Mr. Norton, of Boston (Elect. World., N.Y., May 23, ’96) also stated that the heart could be seen in faint outline, and also its pulsations. The lungs were very transparent. The liver being quite opaque, its rise and fall with the diaphragm was plainly followed. Others have suggested drinking an opaque (to X-rays) liquid, like salt water, and tracing its path.

143. Propagation of X-rays through Air to Distances of 60 Ft.—In Roentgen’s first experiments, the maximum distances at which the fluorescent screen was excited was about 7 ft. Tesla obtained similar action at a distance of over 40 ft. Photographic plates were found clouded if left at a distance of 60 ft. for any length of time. This trouble occurred when some plates were in the floor above and 60 ft. distant from the discharge tube. He attributed the wonderful increase largely to the employment of a single electrode discharge tube, because the same permitted the highest obtainable E. M. F. that could be desired.

144. X-rays with Poor Vacuum and High Potential.—In the course of Tesla’s experiments, he observed that the Crookes’ phenomena and X-rays could be produced without the high degree of vacuum usually considered necessary, § 118. but by way of compensation, the E. M. F. must be exceedingly high, and, of course, the tube and electrical apparatus substantially of those dimensions involved in Tesla’s work. One must be careful not to over-heat the discharge tube, which is likely to occur by increase of potential. He gave definite instructions for preventing the destruction of the tube by heating, by stating that it is only necessary to reduce the number of impulses, or to lengthen their duration, while at the same time raising their potential. For this purpose, it is best to have a rotary circuit interrupter in the primary instead of a vibrating make and break, for then it becomes convenient to vary the speed of the interrupter, which may be, evidently, so constructed that the duration of the impulses may also be varied, for example, by different sets of contact points arranged on the rotary interrupter, and made of different widths.

145. Detail Construction and Use of Single Electrode Discharge Tubes for X-rays. He pointed out that with two electrodes in a bulb as previously employed by nearly all experimenters, or an internal one in combination with an adjacent external one the E. M. F. applicable was necessarily greatly limited for the reason that the presence of both, or the nearness of any conducting object “had the effect of producing the practicable potential on the cathode.” Consequently he was driven, as he said, to a discharge tube having a single internal electrode, the other one being as far away as required. § 9. In view of his ingenious arrangements of the disruptive coil, and circuits, condensers and static screens for the bulb, he found it immaterial to pay attention to some other details followed by experimenters. For example, it made comparatively little difference in his results whether the electrode was a flat disk or had a concave surface.

The form of tube described by Tesla in full, will hereinafter be alluded to as exhibited in the several figures accompanying this description, and it consisted, therefore, of the long tube “b” made of very thick glass except at the end opposite the electrode “e”, where it was blown thin, p. 149. The electrode was an aluminum disk having a diameter only slightly less than that of the tube and located about one inch beyond the rather long narrow neck n, into which the leading-in wire c entered. It is important that a wrapping w be provided around this wire, both inside and outside of the tube. The sealing point was on the side of the neck. An electric screen has been referred to heretofore. It is lettered s, and was formed of a coating of bronze paint applied on the glass between the electrode and neck n. The screen could be made in other ways, for example, as shown at s, Fig. 2, where it consists of an annular disk behind and parallel to the electrode disk. This ring s in Fig. 2. must be placed at the right distance back of the electrode e, but just how far can only be determined by experience. The unique service of the screen was that of an automatic system for preventing the vacuum from becoming too high by use. The peculiar action was as follows, namely from time to time, a spark jumped through the wrapping w within the tube to the screen and liberated just about enough gas to maintain the vacuum at an approximately constant degree. Another way in which he was able to guard against too high a vacuum, consisted in extending the wrapping w to such a distance inside of the tube, that the same became heated sufficiently to liberate occluded gases. As to the long length of the leading-in wire within a long neck behind the cathode, Lenard found the same to be valuable in conjunction with a wrapping around the wire. With high potential, a spark, at a certain high degree of vacuum, formed behind the electrode, and prevented the use of very high potential, but by having the wire extend far into the tube and providing wrappers, the sparking was much less likely to occur. By proper adjustment as before intimated, Tesla could produce just about enough to compensate for the electrical increase of the vacuum. Another difficulty that presented itself in connection with high E. M. F. was the undue formation of streamers heretofore referred to, apparently issuing from the glass, and so often disabling it. He therefore immersed the discharge tube in oil as pointed out in detail hereinafter. The walls of the tube served to throw forward to the thin glass many of those rays that otherwise would have been scattered laterally. Upon comparing a long thick tube of this kind with a spherical one, the sensitive plate was acted upon by the rays in 1/4 the time with the tube. A modification consisted in surrounding a lower portion of the tube, with an outside terminal e, indicated in dotted lines in Fig. 1. In this way the discharge tube had two terminals. The greatest advantage probably in using a long tube, was that the longer it was, within the proper limits, the greater the potential which could be applied with advantages. As to the aluminum electrode, he noticed that it was superior, in comparison with one made of platinum which gave inferior results, and caused the bulb to become disabled in an inconveniently short period of time.

146. Percentage of Reflected X-rays. He performed some preliminary experiments, testing roughly as to whether any appreciable amount of radiation could be reflected or not from any given surface. Within 45 minutes he was enabled to obtain clear and sharp sciagraphs of metal objects, and the same could have been obtained only by the reflected rays, because he screened the direct rays by means of very thick copper. By a rough calculation he found that the percentage of the total amount of rays reflected was somewhere in the neighborhood of 2 per cent.

Prof. Rood, of Columbia University, N.Y., (Sci., Mar. 27, ’96.) by means of an experiment with platinum foil, § 80, concluded that the per centage was about .005, the incident angle being 45 degrees. He regarded this figure as the mere first approximation. Judging from Roentgen, § 85, Tesla, Rood and others, therefore, it seems to be established that the percentage of X-rays reflected is very small.

Prof. Mayer, of Stevens Institute, (Science, May 8, ’96,) is of the opinion that there is a regular or specular reflection, having witnessed some experiments obtained by Prof. Rood, of Columbia Univ., N.Y. Prof. Mayer reported that the original negatives were taken in such a way as to substantiate regular reflection, and were carefully examined by six eminent physicists at the National Acad. of Sci. at Washington, April 23, ’96, and none had the slightest doubt concerning the completeness of the demonstration. The material employed for reflecting was platinum foil. § 103a.

Difference Between Diffusion and Reflection. Judging from the experiments above related, as well as those considered in § 103a, there might at first appear to be contradictory results, reported by different authorities. Experts, it is thought will, without argument, discover the harmonious agreement, and will commend the work of scientists, who, in different parts of the world, and at about the same time, made similar experiments, which now being considered jointly, are found to agree so wonderfully closely. Upon reading the above sections and those referred to, there can be no doubt whatever but that X-rays, upon striking a body are, to a certain per cent. scattered, or thrown back, or bent from their straight course, and sent in a backward and different direction, at one angle or another. The only apparent absolute contradiction to this is that of Perrin, § 103a, near the end. But his is a case of one witness against scores, and, therefore, evidence based upon his experiments, must be counted out. The error was either due to some oversight of his own, or more probably the mistake is merely a typographical one, for often a mistake creeps in between a man’s dictation and the printed work. It is difficult to accuse Perrin of a mistake, for he is a great French authority in such matters. Assuming that no error has occured, let it be noticed that he does not pronounce non-reflection from all substances, but only from steel p. 154, l. 9, and flint, which have been so polished as to form a mirror-like surface, whereas all other experimenters, with scarcely an exception, have not employed such surfaces. The difficult point to believe is that, after six hours, no energy from the mirror could be collected. If we accept Perrin’s results it must be only in regard to those two particular materials, polished steel and flint. Another feature which is on the edge of conjecture, is that of true or specular reflection, referred to by Prof. Mayer, § 146. Many attempts have been undertaken to prove whether the rays were thrown backward on the principle of reflection as light from a mirror, or of diffusion as light from chalk. Let the student notice that the evidence is overwhelming in favor of the turning back of the rays to a very small per cent. upon striking any object. As to specular reflection, which means similar to the reflection of light from a polished mirror, it is practically the same as diffusion, the difference being substantially of a technical nature. This allegation is based upon the detail distinction between reflection and diffusion given by P. G. Tait, professor of natural philosophy, Univ. of Edinburgh, who states, in Encyclo. Brit., vol. 141, p. 586:—

“It is by scattered light that non-luminous objects are, in general, made visible. Contrast, for instance, the effect when a ray of sunlight in a dark room falls upon a piece of polished silver, and when it falls on a piece of chalk. Unless there be dust or scratches on the silver, you cannot see it, because no light is given from it from surrounding bodies except in one definite direction, into which (practically) the whole ray of sunlight is diverted. But the chalk sends light to all surrounding bodies, from which any part of its illuminated sides can be seen; and there is no special direction in which it sends a more powerful ray than in others. It is probable that if we could, with sufficient closeness, examine the surface of the chalk, we should find its behavior to be in the nature of reflection, but reflection due to little mirrors inclined to all conceivable aspects, and to all conceivable angles to the incident light. Thus scattering may be looked upon as ultimately due to reflection. When the sea is perfectly calm, we see it in one intolerably bright image of the sun only. But when it is continuously covered with slight ripples, the definite image is broken up, and we have a large surface of the water shining by what is virtually scattered light, though it is really made up of parts each of which is as truly reflected as it was when the surface was flat.”

146a. Reflected and Transmitted X-rays Compared.—In order to carry on a series of investigations, Mr. Tesla constructed a complete special apparatus represented in Fig. 2, p. 149, and embodied in it also an idea which he attributed to Prof. William A. Anthony, which consisted in arranging for sciagraphs to be produced by the rays transmitted through the reflecting substance as well as by the reflected rays themselves. The figure serves to show at a glance the construction and, therefore, the explanation need be but brief. It consisted of a T tube, having three openings, those at the base and side being closed by photographic plates in their opaque holders, which carried on the outside the objects o and o´ to be sciagraphed. At an angle to both plates, and centrally located, was a reflecting plate, r, which could be replaced by plates of different materials. At the upper opening of the plate B was a discharge tube, b, placed in a heavy Bohemian glass tube, t, to direct the scattered rays downward as much as possible from the electrode, e, to and through the thin end of the discharge tube. The objects to be sciagraphed, namely o and o´, were exact duplicates of each other. No statement could be found as to the thickness of the tested plates, r, except that they were all of equal size. The distance from the bottom of the discharge tube to the reflecting plate, r, was 13 inches, and from the latter to each photographic plate about 7 inches, so that both pencils of rays had to travel 20 inches in each instance. One hour was taken as the time of exposure. After a series of experiments with a great many different kinds of metals, they arranged themselves as to their reflecting power, in an order corresponding to Volta’s electric contact series in air. § 153. The most electro-positive metal was the best reflector, and so on. For exhaustive investigations upon the discovery of Volta, see “Experimental Researches” of Kohlrausch, Pogg. Ann., ’53, and Gerland, Pogg. Ann., ’68. The metals Tesla tested were zinc, lead, tin, copper and silver, which were, in their order, less and less reflecting, and the order is the same in the electro-positive series, zinc being the most positive, and the others less and less so, in the order named. For a complete list of the metals arranged by the Volta series, see any standard electrical text-book. He could not notice much difference between the reflecting powers of tin and lead, but he attributed this to an error in the observation.

He tried other metals, but they were either alloys or impure. Those named in the list above were the pure metals. However, he carried on experiments with sheets of many different substances, and arrived at the following table, which shows particularly the relative transmitting and reflecting powers of the various substances in the rough.

By comparing, as in previous experiments, the intensity of the photographic impression by reflected rays with an equivalent impression due to a direct exposure of the same bulb and at the same distance, that is, by calculations from the times of exposure under assumption that the action upon the plate was proportionate to the time, the following approximate results were obtained:

He stated that while these figures can be but rough approximations, there is, nevertheless a fair probability that they are correct, in so far as the relative values of the sciagraphic impressions of the various objects by reflected rays are concerned.

In order to devise means for testing the comparative reflecting power in a more decided manner, he laid pieces of different metals side by side upon a lead plate. Consequently the reflecting surface was formed of two parts corresponding to the two metals. § 80. The vertically perpendicular partition of lead served to prevent the mingling of the rays from the two metals. Ingenious precautions were taken; as for example, so arranging matters that upon equal areas of the two plates, equal amounts of X-rays impinged. § 80. He undertook to determine the position of iron in the series by thus comparing it with copper. It was impossible to be sure which metal reflected better. The same regarding tin and lead and also in reference to magnesium and zinc. Here, a difference was noticed, namely that the magnesium was a better reflector.

He has made practical application of the power of the substances to reflect a certain per cent. of the rays by employing reflectors for the purpose of reducing the time required for exposure of the photographic plates. It admits, he stated, of the use of reflectors in combination with a whole set of discharge tubes, whereby rays which would be otherwise scattered in all directions are brought more nearly to a single direction of propagation.

It might be argued, that in as much as zinc would reflect only about three per cent. of the incident rays, no practical gain would ensue in sciagraphy by the use of a reflector. He pointed out the falsity of such an argument. In the first place, the angle employed in these tests was 45°. With greater angles, the proportion of reflected rays would be greater assuming that the law of reflection is the same as that of light. By mathematical calculation and tests, he showed that there was no doubt whatever about the advantage of using reflectors. He obtained a sciagraph, on a single plate, of the ribs, arms and shoulder, clearly represented. He stated the details as follows. “A funnel shaped zinc reflector two feet high, with an opening of five inches at the bottom and 23 inches at the top, was used in the experiment. A tube similar in every respect to those previously described, was suspended in the funnel, so that only the static screen of the tube was above the former. The exact distance from the electrode to the sensitive plate was four and one-half feet.”

147. Discharge Tube Placed in Oil.—When the E. M. F. was increased, by having the discharge tube, as usual, in open air, sparks formed behind the electrode, and within the vacuum, and endangered the life of the discharge tube. He obviated this difficulty partly by having the electrode located well within the evacuated space, so that the wire leading to it was unusually long. By excessive E. M. F., also, streamers broke out at the end of the tube. To overcome all difficulties in connection with sparking and breaking of the tube, he followed the proposition of Prof. Trowbridge, and submerged the discharge tube in oil, § 11, at end, and § 13, which was continually renewed by flowing into and out of the vessel in which the discharge tube was contained, all as shown in the accompanying figure, p. 157, “Discharge Tube Immersed in Oil.” The discharge tube, t, may be recognized by its shape, and it is located horizontally in a cylindrical tube lying sidewise upon a table. To regulate the flow of the oil, the reservoir may be raised and lowered by a bracket, s. The X-rays enter the outside atmosphere by passing first through glass, then oil, and then through a diaphragm of “pergament” forming the right hand end of the oil vessel. When the results were compared with those obtained by Roentgen in his first experiments, the rays were found so powerful that it is not surprising that Tesla was able to obtain more definitely a closer knowledge of the properties of the rays. Roentgen obtained, with his tube and a screen of barium platino cyanide, a shadow picture of the bones of the hand at a distance of less than 7 ft., while Tesla obtained a similar picture with a screen of calcic tungstate, and with his tube immersed in oil at a distance of 45 ft. Tesla also made sciagraphs with but a few minutes’ exposure at a distance of 40 ft., by the help of Prof. Henry’s method, i.e., with the assistance of a fluorescent powder. § 151. He referred also to Salvioni’s suggestion of a fluorescent emulsion. He attributed to Mr. E. R. Hewitt the conjecture that the sharpness of the sciagraphs might be increased by a thin aluminum sheet having parallel groves. Several experiments were made, therefore, with wire gauze, as well as with a screen formed of a mixture of fluorescent and iron-fluorescent powders. With the strong power of the rays as obtained by Tesla in combination with such adjuncts, the shadows were sharper, although the radiation, of course, was weakened by the obstruction. § 107b.

With the apparatus involving the discharge tube in oil, and with tremendously high potential, he obtained what may be called wonderful results; for with the sciascope he obtained shadow pictures of the vertebral column, outline of the hip bones, the location of the heart (and later detected its pulsations), ribs and shorter bones, and, without the least difficulty, the bones of all the limbs. More than this, a sciagraph of the skeleton of the hand was perceived through copper, iron or brass very nearly 1/4 inch thick, while glass 1/2 inch thick scarcely dimmed the fluorescence. The skull of the head of an assistant acted likewise, while at a distance of three feet from the discharge tube. The motion of the hand was detected upon the screen although the rays first passed through one’s body. In making observations with the screen, he advised that experimenters should surround the oil box closely, except at the end, with thick metal plates, to prevent X-rays from coming in undesired directions by reflection from different objects in the room. Obviously the shadows will be sharper.

148. Bodies Not Made Conductors by X-rays. Tesla referred to Prof. J. J. Thomson as having announced some time ago “that all bodies traversed by Roentgen radiations become conductors of electricity.” The author has witnessed other similar expressions giving credit to Thomson in this respect, but he understands that Prof. Thomson, having discovered that X-rays discharge both negatively and positively charged bodies, conjectured or drew a corallary as to the probability of the bodies therefore becoming conductors. Tesla, nevertheless, seems to have proved that the corallary does not hold. In the first place he employed the very powerful rays, and next, he let the oil be the substance traversed by the rays. Besides this, he applied a sensitive resonance test. See detail treatment of his experiments on this subject in Elect. Rev., N.Y., June 24, ’93, p. 228. In brief “a secondary 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 considerable variations of potential of the latter. Placing a tube in a box of wood filled 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 any appreciable extent. I then changed the conditions so that the bulb became very active in the production of the rays.”

According to the corallary above referred to, the oil should be, with such an environment and under such subjection, a conductor of electricity, but it was not. He emphasized his satisfaction in the results by saying “the method I followed is so delicate that a mistake is almost an impossibility.”

Prof. W. C. Peckham, Elect. World, N.Y., May 30, ’96, reasoned that the oscillating electro-static action upon the outside of the tube, is concerned in the production of fluorescence, and other properties of X-rays. “These oscillations are certainly synchronous with the vibrations of the cathode rays in the tube, which in turn synchronize with the oscillation in the induction coil. If the vibrations of the tube cannot keep time with those of its coil, few or no X-rays will be given out. The cause seems to be similar to that of sympathetic vibrations in sound. In a word, the discharge tube is a resonator for its coil, and when the coil and tube are properly attuned, the maximum effect is obtained.

149. Appleyard’s Experiment. Non-conductors Made Conductors by Current. Proc. Phil. So., May 11, Nature, Lon., May 24, ’64, p. 93. A piece of celluloid was pressed between two metal plates serving as terminals. A galvanometer was employed to indicate the diminution of resistance by time, and it also showed that the electrification was negative. When mercury was one of the metals, the abnormal results did not occur, except to a very small extent. When the celluloid was replaced by gutta percha tissue, the electrification was normal. Many non-metals were employed, and several were lowered in resistance.

149a. Resistance Somewhat Independent of Metal Particles.—Through a mixture of conducting and non-conducting materials, like a sheet of gutta percha, having brass filings imbedded therein,—with 750 volts, no current passed, and this held true until the proportion in weight of the metal to the gutta percha was 2 to 1. Let it be remembered, also, that selenium is reduced as to resistance under the influence of light.

150. Minchin’s Experiment. Resistance Lowered by Electro-magnetic Waves. Nature, Lon., May 24, ’94, p. 93.—Referring to Appleyard’s experiment, it will be noticed that he found that mixtures of certain limited per cents. of metallic particles and insulators were exceedingly high in resistance. Prof. G. M. Minchin found that such materials became conductors under the influence of powerful electro-magnetic disturbances, and that after the current was conducted, its resistance remained greatly lowered in behalf of very weak impulses, although, before the experiment, the resistance was so high. § 14a. But after the current was interrupted by moving the terminal away from the mixture, the high resisting power returned slowly, at a rate somewhat in proportion to the hardness of the mixture. The film employed consisted of shellac or gelatine or sealing wax, while among the metals was pulverized tin. In the latter case, the resistance was reduced by the electro-magnetic waves from apparent infinity to 130 ohms, the electrodes being displaced by 1 cm.