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 448 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the X-rajs, similar to the action which has been observed at lower volt- 

 ages ? * We connected to the air pump, at the same time, two exactly 

 similar tubes, one of which had two pointed terminals of platinum, the 

 other two pointed terminals also ; but one consisted of aluminium and 

 the other of platinum. The discharge from a Ruhmkorf coil was sent 

 through these tubes, which were in multiple circuit. At a certain stage 

 of the exhaustion it was seen that the discharge passed more easily when 

 the aluminium wire was made a cathode than when it constituted the 

 anode. When the wire terminals in both tubes were made of thin disks, 

 the difference was less marked. This might have been surmised, from 

 previous investigations on the effect of form of electrodes on resulting 

 polarization. t It may be that the anomalous action of aluminium in re- 

 spect to X-rays is due to a species of dielectric polarization on the sur- 

 face of the platinum, and that thus the surface becomes a new source of 

 electrostatic stress, similar to that which was observed by connecting a 

 bit of tin-foil and a capacity with the tube. Since we are dealing with 

 very high differences of potential and with high charges on the ions, the 

 instantaneous exhibition of electrical energy is very great, and might 

 probably explain the diffusion of this energy through the air. Accord- 

 ing to this hypothesis the light manifestations of the X-rays arise only 

 at the fluorescent screens, or at other suitable surfaces. 



In continuing these studies of the electrical discharge in a vacuum, 

 one is naturally led to examine the breaking down of the resistance of 

 rarefied spaces by means of the X-rays which has been noticed by many 

 physicists. The form of tube employed in the first experiment is shown 

 in Figure 10, in which K is the cathode, A the anti-cathode, and C the gap 

 between two terminals connected through a galvanometer with a battery. 

 At the X-ray vacuum the smallest voltage which could set up a current 

 when the gap C was illuminated by the X-rays was 240 volts. In the 

 next form of tube which was employed (Fig. 11), the gap Cwas replaced 

 by the gaps GG' placed on opposite sides of the anti-cathode and very 

 near it. The breaking down effect was strong in this form of tube ; and the 

 residual effect of the X-rays was very marked after they had ceased to 

 be excited. For instance, if the battery current would just pass when 

 the X-rays were illuminating the tube, it would also flow some seconds 

 after the cessation of the X-rays, showing the residual effect of these 

 rays. In order to form an estimate of the resistance offered to disrup- 



* L. Graetz, Wied Ann., No. 10, 1897, p. 323. 



t Karl Robert Klein, Wied. Ann., No. 10, 1897, p. 259. 



