284 Variations observed in Electroscopic Measurements. 



the next made in accordance with (1) lay on the opposite 

 side of the curve, while further measurements made by (1) 

 caused points to resume their place on the smooth curve. In 

 fig. 3, the points obtained by (2) and (3) are marked. The 

 points not marked were obtained by (1). 



The reason for the existence of these effects was sought 

 next. It was found that the placing of an earthed ring round 

 the sulphur, as employed by C. T. R. Wilson in his original 

 work on the subject of electroscopes (Proc. Roy. Soc. 1901, 

 lxviii. p. 152), minimized the effect. Leaf systems, however, 

 were made up with weighted leaves and no earthed ring, 

 which gave the effect considerably less than other leaf systems. 

 It was found eventually that only the leaf systems containing 

 a sulphur rod from 2*5 to 4*4 cm. in length gave the effects, 

 leaf systems with a small bead of sulphur only, with or with- 

 out the earthed ring, hardly showing the effect at all. A 

 weighted leaf system with a rod of sulphur 3'5 cm. long as 

 an insulator was made up, the sulphur being completely sur- 

 rounded by an earthed cylinder of brass. The varying-leak 

 effects were not shown. On raising the ring slightly so that 

 part of the sulphur was left bare the effects were still not 

 shown. On the ring being raised, however, so much that the 

 whole of the sulphur rod was exposed, the effects made them- 

 selves felt at once. The effects are thus due to the presence 

 of a long rod of sulphur, the longer the rod, the more 

 pronounced being the effects, and is favoured by a high 

 potential on the leaf. They can be minimized by using a 

 small bead only, and with this an earthed tube is hardly 

 necessary. The effect can be eliminated by thorough ioniza- 

 tion of the gas in the electroscope between the measurements. 

 In taking decay curves, or absorption curves, when the strength 

 of the source varies over a wide range, the effects described are 

 especially to be avoided. Apart from them the instrument 

 appears to measure reliably and proportionately the whole 

 range of rates of leaks it is possible to time accurately with 

 a fairly high magnification. 



I take this opportunity of thanking Mr. Soddy for his 

 kindness throughout the course of the work and for many 

 valuable suggestions. 



Physical Chemistry Laboratory, 



University of Glasgow, 



December 1909.' 



