DowLiNG & Haughey — Electrification, Phosphorus Smoke Nuclei. 51 



horizontal velocity (v) : thus l//i is a measure of. v. Since the velocity v is 

 probably proportional to the force acting, which, in turn, is proportional to 

 the product of the charge (q) and the electric field, it follows that — 



1/h 00 qV, 



where V is the potential difference between the plates. Hence, observations 



of h should afford us information as to the charge q, corresponding to different 



values of V. 



We found, however, that accurate observations were possible over only a 



limited range of potentials, since, by reason of the dependence of q on V, h 



varied rapidly with V. 



IV. — To evade this difficulty we planned the apparatus shown in fig. 1. 



In this the charge on the smoke is due to the field between the phos- 

 phorus ( P) and the centre plate (£) ; while 



the rise h observed takes place with the 



smoke exposed to the field between the a n p Fi n 1 



plates i? and C. The central partition (B) 



was furnished with a wire gauze window 



through which the smoke stream passed. 



This central plate was earthed. The plate A, 



in front of which the phosphorus was 



supported on a little shelf, was connected 



to a suitable potential dividing device which 



enabled the " charging " potential Vi to be 



varied as desired. The plate C was connected 



to a steady source of potential of the opposite 



sign. 



The plates, about 60 cms. high, were 



contained in a box 13 ems. deep, the front 



and back being of plate-glass fitting close to the edges of the plates. 



A and C were about 10 cms. apart, and B was usually about 1 cm. from A. 



An intense parallel beam of light was directed into the top of the apparatus 

 from the front, and then cast vertically downwards by a right-angle prism 

 within. The consequent slight heating of the glass front at the top was 

 found to set up a convection air-current inside of about the right intensity, 

 and this became sufficiently steady an hour or so after lighting the lamp. 

 The glass back of the apparatus was kept cool by a constant stream of cold 

 water. . The interior was painted dead, black. Two fine silk fibres were 

 stretched, vertically, between ^ and C, close to the smoke stream.. The 

 difference in vertical height where the smoke passed these was observed from 



[5*] 



H m \k 



