for AtniospJterio Electrlcitij 5 



top of one of the ends of the box, and covered over with a piece 

 of silvered glass, upon which a fine vertical scratch — to serve 

 as a slit — had been made. A lens, which could slide upon 

 a rod inside the box, was employed to project the light upon 

 the electrometer mirror, whence after reflection it was returned 

 to the same end of the box as the slit, but at a lower level, 

 and fell upon the horizontal slit in the case of the recording 

 cylinder. By this means a point of light impinged upon the 

 bromide paper, and as the latter rotated, traced out the curve 

 which appears after developing the paper in the usual manner. 

 Fig. 3. 



To Collector 



Fig. 4. E electrometer. R recording apparatus. .S' slit. 



(4) Charging battery. In using the electrometer the opposite 

 pairs of quadrants were kept charged to a fixed potential by means 

 of a battery of the small Leclanche cell used for " flash " lamps. 

 These cells are sold in sets of three and a batch of eight, giving 

 about 35 volts, is quite sufficient for atmospheric observations. The 

 centre of the battery was earthed. The complete apparatus is 

 shown in Fig. 4. 



(5) The Collector. This consisted of a small plate of brass coated 

 with a radioactive preparation. The plate was fixed in the centre 

 of a very short piece of brass tubing and the open ends of the 



