Mr Richardson, On an attempt to detect radiation, etc. 169 



storage cells and a spark-gap of three centimetres was generally 

 used. In this way the spark could be kept going almost con- 

 tinually for hours. The apparatus was set up in a dark room and 

 in addition to this the excited wire was enclosed in a blackened 

 wood box, inside which the photographic plates were exposed. 



In the first experiments a brass rod about 1 cm. thick was used, 

 but this was soon replaced by a brass wire '7 mm. in diameter. 

 Aluminium and copper wires of about "6 mm. and a fine steel 

 wire of "09 mm. diameter were also tested. Exposures of 5, 10, 20 

 and 60 minutes were given successively to different plates. At first 

 the plates were about 2 cms. from the wire, and it was expected 

 they would be more fogged along the line of projection of the 

 w T ire on the plate than elsewhere. The fog observed on developing 

 was however quite uniform and so further exposures were given 

 with plates covered by thick copper with a square hole in the 

 middle. Nothing was observed except when the plate was close 

 up to the wire, when the outline of the square was distinctly seen 

 on developing. This was ultimately shown to be due to a very 

 faint glow between the wire and the edge of the copper. Since, 

 if the rays were very soft ones, they might have been absorbed 

 by 1 — 2 cms. of air at ordinary pressures, the copper plate was 

 now removed and the photographic plate brought up close to the 

 wire. When the wire was only "3 — '5 mm. from the plate, an 

 appearance similar to that produced by drawing an interrupted 

 straight line on blotting paper was obtained on developing. 



The interruptedness is due to the fact that the wire could not 

 be made quite straight on account of its stiffness ; so that it was 

 hearer the plate in some parts than others. The edges of the 

 linear spots presented in places a peculiar jagged appearance when 

 magnified. The nature of the phenomenon was investigated by 

 inserting thin plates of various material between the wire and the 

 photographic plate. The action was found to be transmitted 

 through transparent substances {e.g. thin glass and mica) but 

 was stopped by the thinnest aluminium foil. It was traced 

 finally to a very faint glow which occasionally passed along the 

 wire. The effect seemed identical in every respect with that 

 recently investigated by M. T. Tomassina 1 . It was found im- 

 possible to make sure of this glow never occurring, so that the only 

 thing left was to try for an effect at low pressures when any 

 radiation which might be given off would not be absorbed by 

 the air. 



In the form of apparatus used at first, the excited wire simply 

 passed in at one end of a wide tube which could be exhausted 

 and out again through a short side tube fixed at right angles to 



1 Comptes Rendus, May 1900. 



