Roentgen Rays and Phenomena of the Anode and Cathode.

by Edward P. Thompson

Tesla’s Figs. 1 and 2, Reflection and Transmission of X-Rays by Different Substances. [§ 145] and [§ 146a].

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.