Dr. Roentgen Continues His Investigations and Discovers That X-Radiation Causes Air to Become Electrically Conductive


Editor's Note: Following his discovery of x-radiation a few months ago, Dr. Roentgen has continued his investigations to develop a better understanding of the properties of the new kind of radiation. One of his most significant recent observations is that when x-radiation passes through the air, the air becomes an electrical conductor.

We can expect that in the future this property might become one of the most effective ways of measuring x-ray exposure and will lead to the characterization of x-rays as an ionizing radiation.

The details of Dr. Roentgen's findings are described in his second communication which he released on March 9, 1896, and is printed below.


W. C. Roentgen: On a New Kind of Rays

(Continued)

Since my work must be interrupted for several weeks, I should like to present at this time some new results in the following.

18. At the time of my first publication I knew that x-rays are able to discharge electrified bodies, and I suspect that in Lenard's experiments it was also the x-rays and not the cathode rays, transmitted unchanged by the aluminum window of his apparatus, that produced the effects upon electrified bodies at a distance. However, I waited until I could present incontestable results before publishing my experiments.

These seem to be obtainable only if the observations are made in a room that not only is protected completely from the electrostatic forces emanating from the vacuum tube, from the conducting wires, from the induction apparatus, and so forth, but also is closed against air that comes from the region of the discharge apparatus.

Accordingly I had a box built of zinc plates soldered together, which is large enough to accommodate me and the necessary instruments and which is completely airtight with the exception of an opening that could be closed by a zinc door. The wall opposite the door is to a large extent covered with lead; at a place near the discharge apparatus, which is set up outside the box, an opening 4 cm wide is cut out of the zinc wall and its lead cover, and this opening is in turn made air-tight with a thin sheet of aluminum. Through this window the x-rays can enter the observation box.

Now I observed the following:

(a) Positively or negatively electrified bodies set up in air are discharged if they are irradiated with x-rays; the more intense the rays, the more rapid the discharge. The intensity of the rays was estimated by their effect upon the fluorescent screen or upon a photographic plate.

Generally it is immaterial whether the electrified bodies are conductors or insulators. Moreover, so far I have not been able to find a specific difference in the behavior of different bodies with regard to the rate of discharge, nor in the behavior of positive and negative electricity. Yet it is not impossible that small differences exist.

(b) If an electrified conductor is not surrounded by air but by a solid insulator, e.g., paraffin, the irradiation of it has the same effect as moving a grounded flame over the insulating cover.

(c) if this insulating cover is surrounded by a tight-fitting grounded conductor, which like the insulator must be transparent to x-rays, the radiation exerts upon the inner electrified conductor no effect detectable with the available means.

(d) The observations cited under a, b, c indicate that air that is irradiated with x-rays has acquired the property of discharging electrified bodies with which it comes in contact.

(e) If this is really the case and in addition if the air retains this property for some time after being exposed to x-rays, it should be possible to discharge electrified bodies that themselves are not directly irradiated by x-rays simply by conducting irradiated air to them.

One can be convinced of the validity of this conclusion in different ways. I should like to describe one experimental set-up, although it is not the simplest one.

I used a brass tube 3 cm wide and 45 cm long; a few centimeters from one end of the tube, part of its wall was cut away and replaced with a thin sheet of aluminum; through the other end a brass sphere, fastened to a metal rod and insulated, was sealed air-tight into the tube. Between the sphere and the closed end of the tube there was soldered a little side tube, which could be connected to an exhaust apparatus; when suction was applied, air that passed the aluminum window on its way through the tube flowed around the brass sphere. The distance from window to sphere was over 20 cm.

I set this tube up inside the zinc box so that through the aluminum window of the tube the x-rays could enter perpendicularly to its axis and so that the insulated sphere lay in the shadow beyond the range of these rays. The tube and zinc box were connected to each other; the sphere was connected to a Hankel electroscope.

It was then observed that a charge either positive or negative given to the sphere was not influenced by the x-rays as long as the air remained at rest in the tube, but that at once the charge decreased considerably if irradiated air was drawn past the sphere by strong suction. Then a constant potential from a storage battery was applied to the sphere and when irradiated air was continuously sucked through the tube, an electric current was produced just as if the sphere had been connected to the tube wall by a poor conductor.

(f) The question arises in what manner air can lose the property given to it by x-rays. Whether in time it loses the property itself, that is, without coming in contact with other bodies, is still unsettled. However, it is certain that a brief contact with a body that has a large surface and is not necessarily electrified may render the air ineffective. If, for example, one placed a sufficiently large stopper of cotton so far into the tube that irradiated air must pass through the cotton before it reaches the electrified sphere, the charge of the sphere remains unchanged; even while suction is applied.

If the stopper is placed in front of the aluminum window, one obtains the same result as without cotton: a proof that dust particles cannot possibly be the cause of the discharge observed.

Wire screens have an action similar to cotton; however, the screen must be very fine, and many layers must be put on top of one another if the irradiated air passing through them is to be made ineffective. If these screens are not grounded, as has been assumed so far, but are connected to a source of electricity of constant potential, the observations have always been what I anticipated; however, these experiments have not yet been completed.

(g) If the electrified bodies are placed in dry hydrogen instead of air, they are also discharged by x-rays. It seemed to me that the discharge in hydrogen proceeded somewhat slower; however, this is still uncertain because of the difficulties in obtaining equal intensities of x-rays in a series of consecutive experiments.

The method of filling the apparatus with hydrogen very likely precludes the possibility that the denser layer of air originally present on the surface of the bodies could play an important role in the discharge.

(h) In highly evacuated spaces the discharge of a body struck directly by x-rays proceeds much more slowly--in one case, for example, about seventy times more slowly than in the same vessels when they are filled with air or hydrogen of atmospheric pressure.

(i) Experiments have been started on the behavior of a mixture of chlorine and hydrogen under the influence of x-rays.

(j) Finally, I should like to mention that one must often accept with caution the results of experiments on the discharging effects of x-rays in which the influence of the surrounding gas has not been taken into account.

19. In some cases it is advantageous to insert a Tesla apparatus (condenser and transformer) between the discharge apparatus, which furnishes x-rays, and the Ruhmkorff coil. This arrangement has the following advantages: First, the discharge tubes are less liable to be punctured and heat up less; secondly, the vacuum, at least so far as my home-made tubes are concerned, keeps for a longer time; and, thirdly, some apparatus produce more intense rays. Some tubes that were evacuated too little or too much to work satisfactorily on the Ruhmkorff coil alone functioned satisfactorily with the use of the Tesla transformer.

The question arises--and I should like, therefore, to mention it without contributing anything to its solution at present--whether x-rays can also be produced by a continuous discharge from a source of constant potential or whether fluctuations of the potential are absolutely necessary to produce them.

20. It is stated in paragraph 13 of my first communication that x-rays can be produced not only in glass but also in aluminum. In continuing the investigations along these lines no solid body could be found that was not able to produce x-rays under the influence of cathode rays. I also have found no reason for liquid and gaseous bodies' not acting in the same manner.

However, quantitative differences in the behavior of different bodies have been found. For example, if one lets cathode rays fall upon a plate, one half of which consists of a 0.3 mm platinum sheet and the other half of a 1 mm aluminum sheet, one observes on the photograph of this double plate taken with a pinhole camera that the platinum emits considerably more x-rays from the front side where it has been struck by the cathode rays than the aluminum emits from the same side. From the rear side, however, hardly any x-rays are emitted from the platinum but relatively many from the aluminum. In the latter, rays have been produced in the front layers of the aluminum and have penetrated through the plate.

One can easily arrive at an explanation of this observation, but it might be advisable to learn about some other properties of the x-rays first.

However, it should be mentioned that the observed facts also have a practical significance. According to my experience up to now, platinum is best suited for the production of x-rays of highest intensity. For several weeks I have used with good success a discharge tube with a concave mirror of aluminum as cathode and a platinum foil as anode, which has been placed in the focus of the cathode and inclined 45 degrees in relation to the axis of the mirror.

21. In this apparatus x-rays are emitted from the anode. From experiments made with apparatus of various shapes I must conclude that, insofar as the intensity of x-rays is concerned, it does not matter whether these rays are produced at the anode or not.

Especially for experiments with alternating currents from a Tesla transformer a discharge apparatus is being built, in which both electrodes are concave aluminum mirrors, whose axes form a right angle; in their common focus a platinum plate is placed that receives the cathode rays. A report on the usefulness of this apparatus will appear later.

Finished: March 9, 1896

Würzburg. Physikal. Institut d. Universität.


In the Nov. 1, 1895 edition of The X-ray Century we examined the history of gas discharge tubes.

In the Nov. 8, 1895 edition of The X-ray Century we were there when Prof. Roentgen discovered a new kind of ray.

In the Dec. 1, 1895 edition of The X-ray Century we looked at the investigation which led Dr. Roentgen to write this paper.

In the Jan. 1, 1896 edition of The X-ray Century we read Prof. Roentgen's first paper describing the new kind of ray.

In the Feb. 1, 1896 edition of The X-ray Century we watched the word spread around the world.

In the March 1, 1896 edition of The X-ray Century we saw the first uses of x-rays for diagnostic purposes in several different countries.

In the April 1, 1896 edition of The X-ray Century we looked on as Becquerel discovered radioactivity.

In the May 1, 1896 edition of The X-ray Century we were there as a writer from McClure's Magazine interviewed Dr. Roentgen.


The next edition of The X-ray Century will be published on July 1.


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