OWING to my absence from Cambridge in the Easter vacation, I have not until to day seen the paper by Lord Kelvin, Dr. Beattie and Dr. M. Smolan (NATURE, March 25), on the influence of Röntgen rays on electric conduction through air, paraffin, and glass, in which the authors state that they cannot detect any influence of Röntgen radiation on conduction through solids. I think that the difference between this result and the one obtained by Mr. McClelland and myself arises from the temporary character of the effect of the radiation on solids. The increase in the conductivity of solids is only appreciable for a short time after the application of the electric force (see NATURE, July 30, 1896, p. 306); under long-continued electromotive forces the conductivity seems unaffected by the rays. The effect might perhaps be more accurrately described as an increase in the electric absorption, rather than as an increase in conductivity. I have been for the past few months engaged in experiments on the effect of the rays on solids and liquids, particularly liquids; and, though the experiments have been much interrupted by the pressure of other work, I hope soon to have them ready for publication. There is one experiment, however, which may be of interest. Of all the liquids tried, that sold as vaseline oil has proved the best insulator; in its pure state it is very transparent to Röntgen rays, so to increase the absorption of these rays I stained the oil with iodine, when it became very opaque to them. The oil does not insulate so well after staining as it did before, but the effect of a slight amount of conductivity is not of importance when the following method is used. Three electrodes, A, B, C, are placed in a leaden vessel filled with the oil. B, which is between A and C, is connected to one pair of quadrants of an electrometer, A and C to the terminals of a battery of 1000 small storage cells. If there is any leakage the potential of B will, in general, not remain zero after the battery is put on, but it will do so if an earth connection is made at the proper place in the battery. The base of the vessel below B C was cut out, and an aluminium vessel inserted, so that the liquid between B and C could be exposed to the Röntgen rays. A balance was obtained with the rays off; when the rays were turned on, the potential of B no longer remained zero, but changed in the way it would if the conductivity between B and C had increased. This effect was small but well marked, and seemed to last however long the electromotive force was kept on.
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About this article
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