113. Lodge’s Experiment. X-rays Most Powerful when the Anode is the Part Struck by the Cathode Rays. Pin-Hole Pictures by X-rays to Determine Source of X-rays, and Pin-Hole Images upon Glass Compared. The Elect., Lon. Apr. 10, ’96, p. 784.—The object of the experiment was to confirm, if possible, by a modified construction, the source of the X-rays, as being the surface struck by cathode rays, whether the surface is that of glass or any other substance. He had constructed, for this purpose, a discharge tube, as illustrated in the diagram, which may be seen, at a glance, to contain a concave electrode at one end, and a flat electrode at the other. Between them, and connected to the concave electrode, is an inclined sheet of aluminum, shading both electrodes. The wires leading to the aluminum sheet are well protected by glass. He arranged matters so that either the concave or the flat electrode could be made positive or negative. The operation consisted first in taking through a pin hole, 1/4 of an inch in diameter, X-ray pictures on photographic plates, from different points, at measured distances. After these were taken, glass plates received the luminous images at the positions of the sensitive plate. Pencil drawings were then made, and compared with the X-ray pictures. The experiment involved also the repetition of this operation, except that the polarity of the terminals was changed.
“When the small flat disk was cathode, every part of the complicated anode appeared strongly and quickly on the plate, especially the tilted and first bombarded portion on a photographic plate placed above the tube. The cathode disk itself did not show at all. On a plate placed below the bulb, the anode cup appeared strong, but the tilted disk did not appear. On the other hand, … its focus spot acted as a feeble point source, by reason of a few rays reflected back on to it from the cup.
“When the current was reversed, the small disk anode showed faintly, being excited by rays which had penetrated the interposed tilted disk, but again the cathode hardly showed at all, not even the tilted portion on a plate placed below the bulb. This is confirmed by J. Perrin. In no case could an image of the cathode be obtained. (Comptes Rendus) Mar. 23, ’96. From trans. by L. M. P.) By giving a very long exposure (two hours), some impression was obtained by Dr. Lodge about equal to that from the shaded anode disk; but, of course, if the tilted plate had been under these circumstances an anode, it is well known that a few minutes would have sufficed to show it strong upon the plate beneath.
“Hence, undoubtedly, the X-rays do not start from the cathode or from anything attached to the cathode but do start from a surface upon which the cathode rays strike, whether it be an actual anode or only an ‘anti-cathodic’ surface. Best, however, if it be an actual anode. (Independently discovered by Rowland, [§ 116]. and Roentgen, [§ 91].”)
“When the glass walls, instead of receiving cathode rays, are pierced only by the true Roentgen rays from the disk in the middle, no evidence is afforded by my photograph that the glass under these circumstances acts as a source. It is well that it does not, for its only effect would be a blurring one. [§ 91]. With focus tubes, the glass phosphoresces under the action of the X-rays as anything else would phosphoresce, but its phosphorescence is not of the least use. It is a sign that a tube is working well, and that the rays are powerful; but if by reason of fatigue ([§ 58]) the glass ceases to phosphoresce strongly, the fact constitutes not the slightest detriment.”
X-Ray Uninfluenced by a Magnet. Severe Test.—His first experiment on magnetic deflection, the sciagraph of a magnet with a background of wire gauze, only showed that if there were any shift by reason of passage of rays between the poles it was very small; but he definitely asserted, as in accompanying diagram, that a further experiment has been made which effectually removes the idea of deflectibility from his mind, and confirms the statement of Professor Roentgen. [§ 79]. A strong though small electro-magnet, with concentrated field, had a photograph of its pole-pieces taken with a couple of wires, A and C, stretched across them on the further side from the plate—nearer the source—and a third wire, B, also stretched across them, but on the side close to the plate. These three wires left shadows on the plate, of which B was sharp and definite, while A and C were blurred. Two sciagraphs were taken by Mr. Robinson, one with the magnet on, and one with the magnet reversed. On subsequently superposing the two plates, with the sharp shadows of B coincident, the very slightest displacement of shadows A and C could have been observed, although those shadows were not sharp. But there was absolutely no perceptible displacement, the fit was perfect. Consequently the hypothesis of a stream of electrified particles is definitely disproved as no doubt had already been effectively done in reality by Professor Roentgen himself. But it must be noted, he stated, that the hypothesis of a simple molecular stream—not an electrified one—remains a possibility. The only question is whether such an unelectrified bombardment would be able to produce the observed effects. It must be remembered, Dr. Lodge stated, that Dr. Lenard found among his rays two classes as regards deflectibility—some much deflected, others less deflected; and it must be clearly understood that his deflections were observed, not in the originating discharge tube, where the fact of deflection is a commonplace, but outside, after the rays had been, as it were, “filtered” through an aluminum window. He did not, indeed, observe the deflection in air of ordinary density; it was in moderately rarefied air that he observed it, [§ 72a], but he showed that the variation of air density did not affect the amount, but only the clearness of the minimum magnetic deflection. The circumstance that affected the amount of the deflection was a variation in the contents of the originating or high-vacuum tube.
114. Lodge’s Experiment. Apparatus Employed. The Elect., Lon., April 10, ’96, p. 783.—With his apparatus, he was able to obtain rays sufficiently powerful to illuminate the usual fluorescent screen after passing through one’s skull. It is of interest to note about the details of the electrical apparatus ([§ § 106], [109], [131], [137]) used by those who experimented. The best results were obtained by a make and break of a direct primary current at a point under alcohol, the primary battery consisting of three storage cells, and the current of the primary acting on a large secondary coil. Leyden jars he considered entirely unnecessary, and he preferred direct currents to alternating currents for the primary. He did not give the exact dimensions of the primary and secondary coils, but, judging from reports of others and the author’s own experience, it is highly preferable to have what is called a very large inductorium, 15 in. spark in open air, or else the Tesla system ([§ § 51], [137]). There is little satisfaction in trying to perform the experiments with induction coils adapted to give only a 2 or 3 in. spark in open air.
