The Cathode Rays

Proper Methods of Experimentation — Source and Character of the Rays — Triangulation in Surgery.

NEW HAVEN, March 2.

The American Journal of Science in the number to be published to-morrow will contain articles on the cathode rays by Prof. A. W. Wright of Yale, John Trowbridge of Harvard, and Prof. H. A. Rowland, N. R. Carmichael, and L. F. Briggs of Johns Hopkins.

In the longer article, by Prof. Wright, he gives fully for the first time the methods and mechanism employed. He says:

For the production of the cathode rays, a Crookes tube, nearly spherical in shape, was used, of the kind often employed to exhibit the independence of the negative electrode in high vacuum. It is provided with four electrodes, three of straight wire, of which two are situated at the opposite extremities of a diameter and directed radially, while the third is placed at the end of a diameter at right angles to the line of the other two. The fourth is situated a little to one side of the point opposite the third electrode just described. This disposition is advantageous, as it leaves a space of smooth thin glass opposite the third and fourth electrodes, which were the ones employed, sometimes one, sometimes the other, as the cathode. The fourth wire terminates in a slightly concave disk or cup about two centimetres in diameter. The concave disk concentrates the cathode rays, issuing normally from its surface, at a point near the opposite surface of the tube, sending them through a small area of the glass wall, which becomes strongly heated. The photographic plates show this concentration of the rays plainly. On account of the unequal distribution of the effect, however, as well as because of the inferior definition of the pictures thus obtained, the third straight wire electrode was used in preference. This gave somewhat less intense effects, making a longer exposure necessary, but the results were otherwise much more satisfactory. The induction coil used with the tube gives a spark of six or seven centimetres in air, and was actuated by four, sometimes five, secondary cells.

The development of the negative requires especial care. It is better to use a developer of less than the ordinary strength, and to use no alkaline accelerator at all, until just at the end of the process, and then very sparingly and chiefly for the purpose of intensifying the picture. This makes the process very long and tedious, but the results amply compensate for the labor of obtaining them. Aside from this it appears that the photographic plates which have been subjected to the rays are peculiarly slow, even with the ordinary mode of development. The picture is a long time in making its appearance and grows very gradually. This is perhaps in part due to the fact that the effect of the cathode rays extends through the entire thickness of the film, which must therefore be thoroughly developed to the bottom to bring out the full effect, or possibly to a specific action upon the film itself.

Prof. Wright gives also at length a series of experiments showing the very slight deviation of the rays outside of the tube under a magnet, but a significant though small variation of results in the different experiments. He continues:

An attempt to discover the point of departure of the rays was made by tracing the axes of the trails or pencils until they met upon the plot. In the case of the first two experiments, they were found to come together at a point very near the centre of the spherical body of the vacuum-tube, thus indicating that they left the surface of the glass nearly normally. The third case showed the point of meeting nearer the cathode, but the results of the three pairs of pencils varied much from each other.

If the cause of the excitation is an ordinary induction coil, the resistance of the circuit of the secondary is so great that the oscillations in this circuit at each opening or closing of the primary circuit will be rapidly extinguished, and the mode of discharge will approximate closely to a simple discharge at each movement of the interrupter, producing at the terminal of the electrode within the tube an unsymmetrical rise and fall of the electrostatic potential. This must give rise to the passage of inductive actions through the space surrounding the electrode both within and without the tube. The period or duration and phases of these inductive pulses will be essentially the same as those of the variations of potential at the surface of the electrode, and as these are not a continuous sequence like the elements of a train of waves, the inductive actions in space may be regarded as very probably occurring in the form of isolated or unsystematic pulses, succeeding one another at intervals long compared with their own duration, and more or less irregular with most forms of interrupters used with the coil, from the variations in their action. As to the mode of propagation of these pulses the most natural view would be to regard them as analogous to longitudinal compression waves in an elastic medium, but of an unsystematic character.

In closing, Prof. Wright calls attention to certain phenomena noted in a paper published by him in 1870, giving results suggestive of the present cathode pictures; and also calls attention to his subsequent papers in 1871 and 1875, in which “attention was called to the fact that many cases of images formed upon the bodies of persons killed by lightning, which had been reported by reputable observers, might be explained by supposing that the objects represented were so situated as to interrupt the lines of electrical action.” He concludes:

These facts suggest the probability that every electrical discharge which is not simply a flow of potential along a conductor may produce effects of a similar kind, though much inferior in accuracy of form and dimensions to those produced by the agency of the cathode rays from a high-vacuum tube.

Prof. Trowbridge of Harvard gives a plan for locating by triangulation the exact position of a shot in a human body, giving its depth in the flesh instead of merely its line. He says:

I was led to this conclusion by considering Rumford’s photometer. This instrument, it is well known, consists merely of a vertical rod placed opposite a suitable screen of white paper. The two lights the intensities of which are to be compared are placed in a fixed position, and throw two shadows of the rod on the screen. From a measurement of the positions of the lights when shadows of equal intensity are thrown on the screen, an estimation of the brightness of the lights can be obtained. Moreover, by measuring the distance between the shadows, and by drawing lines from them to the lights, the position of the rod throwing the shadows can be determined. This position is evidently at the intersection of these lines.

I have used two Crookes tubes with two terminals making an angle with each other, and have employed a to-and-fro excitation by means of a Tesla coil. A suitable screen of glass shielded the sensitive plate first from one cathode and then from the other. From the distance between the shadow pictures of a shot, for instance, on the back of the hand, and from the position of the terminals, the height of the shot above the sensitive plate could be estimated. It seems to me that this method promises to be of importance in the surgery of the extremities of the body; for the question whether to make an incision from the palm of a child’s hand or from the back of the hand is an important one. Stereoscopic pictures can also be obtained.

The use of a Tesla coil in obtaining shadow pictures is advantageous in certain respects, for by changing the size of the spark gap in the primary circuit of the Tesla coil one has a great range of electrical energy at command. This range can be still further increased by putting the spark gap in a magnetic field. I have taken such pictures in less than a minute, showing the bones in the fingers. The tubes were at first destroyed by disruptive sparks over the surface of the tube which apparently penetrated the glass between the platinum terminals and the glass. I have lately discovered, however, that if the terminals of the tube are placed in a vessel filled with paraffine oil and if the oil is kept cool by an outside vessel filled with snow or ice, the entire energy developed by the Tesla coil can be employed, and the tubes are not destroyed.

I have tried wooden lenses, both double convex and double concave, in order to see whether the rays travel slower or faster in wood than in air, but my results are negative.

Prof. Rowland and his associates of Johns Hopkins University say of the rays:

So far as yet observed the rays proceed in straight lines, and all efforts to deflect them by a strong magnet either within or without the tube have failed. Likewise prisms of wood and vulcanite have no action whatever so far as seen, and, contrary to Röntgen, no trace of reflection from a steel mirror at a large angle of incidence could be observed. In this latter experiment the mirror was on the side of the photographic plate next to the source of the rays and not behind it, as in Röntgen’s method.

We have, in the short time we have been at work, principally devoted ourselves to finding the source of the rays. For this purpose one of our tubes, made for showing that electricity will not pass through a vacuum, was found to give remarkable results. This tube had the aluminium poles within one millimetre of each other, and had such a perfect vacuum that sparks generally preferred 10 centimetres in air to passage through the tube. By using potential enough, however, the discharge from an ordinary Ruhmkorff coil could be forced through. The resistance being so high, the discharge was not oscillatory as in ordinary tubes, but only went in one direction. In this tube we demonstrated conclusively that the main source of the rays was a minute point on the anode nearest to the cathode. At times a minute point of light appeared at this point, but not always.

Added to this source the whole of the anode gave out a few rays. From the cathode no rays whatever came, neither were there any from the glass of the tube, where the cathode rays struck it, as Röntgen thought. This tube as a source of rays far exceeded all our other collection of Crookes tubes and gave the plate a full exposure at 5 or 10 centimetres in about five or ten minutes with a slow-acting coil giving only about four sparks per second.

The next most satisfactory tube had aluminium poles with ends about three centimetres apart. It was not straight, but had three bulbs, the poles being in the end bulbs and the passage between them being rather wide. In this case the discharge was slightly oscillatory, but more electricity went one way than the other. Here the source of rays was two points in the tube, a little on the cathode side of the narrow parts.

In the other tubes there seemed to be diffuse sources, probably due in part to the oscillatory discharge, but in no case did the cathode rays seem to have anything to do with the Röntgen rays. Judging from the first two most definite tubes, the source of the rays seems to be more connected with the anode than the cathode, and in both of the tubes the rays came from where the discharge from the anode expanded itself towards the cathode, if we may roughly use such language. As to what these rays are it is too early to even guess. That they and the cathode rays are destined to give us a far deeper insight into nature nobody can doubt.