the Point of an Electrified Needle. 



549 



density does not vary between = and 0=-', whilst curve A 



shows a change of density within these limits which is by no 

 means negligible. The value of a in equation (1) is thus 



1 



—. = 1°085, and the values of/ obtained in the previous 



V'85 

 research must be multiplied by that number. 



The curve A in fig. 6 is the density curve obtained with 

 the larger sphere ; the density appears to fall off a little 

 more rapidly, and the correction factor derived from this 

 curve is about 2 per cent, higher than that obtained from the 

 small sphere. The discrepancy is no doubt due to the im- 

 perfectly spherical shape of this sphere. The value of a may 

 therefore be taken as very close to 1*09. 



It was desired in the above determinations to have the 

 needle sufficiently long and at a sufficient distance from the 

 screen to leave the distribution of the charge practically in- 

 dependent of any conditions other than the shape of the 

 needle-point. To ensure that the screen was at a sufficient 

 distance, observations were taken whilst the screen was at a 

 distance of 50 cm., and again when the distance was increased 

 to about 90 cm. ; the two sets of observations were found to 

 agree within the limits of experimental error. Professor 

 Chattock, however, in view of possible future work, wished 

 to obtain the minimum values of the length of needle and 

 distance of screen for which the distribution was still depen- 

 dent only on the shape of the needle. 



The screen was therefore lowered a little at a time, and for 

 each position of the screen the relative density for a certain 

 value of 6 (0=1*047) was ascertained. Though the absolute 















Fig. 



5. 















10 













H 

















'1 







i ; 



































.. «^- 







=&-■ - 



1 



1 . 













If 





v 





























































If i°R 



24 



SoR 



density naturally rose as the distance decreased, the relative 

 density cr remained constant to a certain limit beyond which 



