Candle Power of X-Rays

BY EDWARD P. THOMPSON.

I used the following method in order to obtain a rough idea as to the strength of the Röntgen rays as compared with those of a canale. The process, of course, will not give the true candle-power, for the simple reason that the X-rays are not visible, but nevertheless, what may be termed the equivalent of the candle-power may be measured, so that the result impresses one with the extreme weakness of the rays in one sense, although they have so great a penetrating power.

In order to obtain the best results roughly, I receded from the Crookes' tube while looking into the skiascope, until the luminosity disappeared, and then measured the distance between the screen and the Crookes tube. It was about six feet. This represents about the usual power of X-rays, except in Tesla's results where he obtains fluorescence of calcic tungstate at 40 feet. Of course, the luminous rays of the Crookes tube were extinguished by the double thickness of black paper between it and the fluorescent screen.

I then held the skiascope (the name suggested by Prof. Magie, and which I think is by far the best) so as to face the candle, and I walked away from the same until the luminosity of the screen disappeared. The distance was likewise measured. In this case there should be no opaque substance between the candle and the screen. When it is between the Crookes tube and the screen, it has practically no effect upon the X-rays, whereas it would cut off all the rays from the candle. The distance in the latter case was about 120 feet. If the power of X-rays varies inversely as the square of the distance, a candle is 400 times more powerful in producing fluorescence than the X-rays, particularly from a Crookes tube that I employed and charged with the particular apparatus. In the same manner the candle-power of the X-rays from any Crookes tube could be measured.

It is assumed that the fluorescent screen represented the total energy for producing fluorescence as far as the X-rays were concerned. However, the candle rays are not all converted into fluorescent light; for those below the green, including yellow, orange, red and infra-red rays, produce no luminosity upon the fluorescent screen; therefore this method does not compare all the energy of the X-rays with the energy of the candle rays, but it serves to give a rough comparison. It is assumed that the fluorescent screen converted all the ultra-violet rays, violet, indigo, blue and green rays into luminous rays. This assumption may cause unknown errors in the result.

Neither would a corresponding photographic method be exact, because the silver compound would not be chemically acted upon by the infra-red rays. It should be noticed also that both methods are based upon the assumption that all the rays in both cases are converted into green rays of light by means of the fluorescent screen and into chemical energy by the photographic plate.

It remains a fact, however, that as far as we are able to transform the energies, the X-rays are weak as compared with those from the candle. Both will produce fluorescence and chemical action, but the candle very much more powerfully than the X-rays. When the two sets of rays are compared on other grounds, the Röntgen rays are more powerful, as, for for example, in reducing the sparking distance either for a positively or negatively charged insulated plate. Again, from another point of view, the candle rays can be turned out of their course by reflection or refraction. It still remains true, however, in the rough, that inasmuch as fluorescence and chemical action are both produced by the two forms of energy, therefore, these phenomena serve as a standard upon which to operate in order to obtain what may properly 'be called candle-power, with the restriction that it is not the true candle-power, because things of exactly the same nature are not compared. In the case of the candle-power of the sun, are light, oxy-hydrogen flame, the incandescent lamp, gas flame, etc., their natures are the same as that of the candle, namely, all have all the luminous rays of the spectrum, but some more than others, so that in these cases we are comparing the rays of similar natures.