378 Dr. Willows and Mr. Peck on the Action 



anode. B, C were disks, and either could be made the cathode 

 at will : the limbs containing them were made as nearly as 

 possible equal. At low pressures, when B was cathode, the 

 positive column was very short; when the glass round B gave 

 a green fluorescence scarcely any positive column was present. 

 Under similar conditions with C cathode the column extended 

 to the side limb. With the field on at either cathode, the 

 positive column extended nearly the whole length of the tube. 

 In spite o£ this great difference in development, no difference 

 could be found in the behaviour of B or C with the magnet, 

 at a pressure when the latter brought about a large decrease 

 in the voltage. 



We may therefore conclude that the fall in potential has 

 its chief seat near the cathode. 



As to the cause of this fall, it may be suggested that the 

 negative ions are caused to travel over greater paths in the 

 dark space, because the magnetic field causes them to move 

 obliquely through it. In this space, therefore, they produce 

 more ions by collision, and the discharge being previously poor 

 in ions at this point, as shown by numerous experiments, 

 this causes the fall in voltage between the terminals. 



Sir John Cass Technical Institute, 

 Jewry St., E.C. 



XXXVI. Action of Radium on the Electric Spark. By R. S. 

 Willows, M.A., D.Sc, and J. Peck, B.Sc* 



IT is well known that radioactive substances, w r hen placed 

 near a spark, in general cause the discharge to pass 

 more readily by the formation of ions in the electric field. 

 The action on a long spark is, however, different, and appears, 

 as far as our knowledge goes at present, somewhat irregular. 

 In some cases the discharge passes more readily, in others 

 with greater difficulty f . 



The present paper describes experiments which were 

 undertaken with the object of studying the origin of this 

 influence. 



The spark was produced between two brass spheres of 

 unequal diameters, one 27 mm. the other 48 mm., by means 

 of a Wimshurst machine carrying three pairs of plates of 2 

 feet diameter. The radium bromide, 5 mgm., was contained 

 in a small capsule closed with mica. The strength of the 



* Communicated by the Physical Society: read January 27, 1905. 



t See abstract of a paper by D. M. Sokoltzow, ' Science Abstracts,' 

 vol. vii. p. 343 (1904), and also one by A. Stefanini & L. Magri on the 

 same page. 



