544 Mr. F. B. Young on the Field at 



e> 



supported and insulated with paraffin-wax. Directly oppo- 

 site C was a slit D, about 4 cms. in length. To the plug, 

 which served as a proof-plane, was soldered a fine copper 

 wire E which passed through D to an electrometer. The 

 wire was insulated with rubber tubing, which was surrounded 

 by an earth-connected sheath of tinfoil ; it was held in posi- 

 tion by means of a plastic mixture of paraffin- wax and Prout's 

 glue with which the rubber tubing was filled. This arrange- 

 ment served to make the electrostatic capacity of the wire 

 constant and independent of its position relative to the 

 cylinder A. On the great circle passing through C and D 

 was scratched a scale of centimetres and with the aid of a 

 cardboard gauge which marked the position of the extremity 

 of the needle's axis the proof-plane could be moved, by 

 rotating the sphere in its socket, to a known distance from its 

 central position. 



The mode of employment of this arrangement may be 

 briefly stated as follows: — a charge was induced on the needle- 

 point by charging the screen to a certain constant potential 

 and the portion of the charge residing on the proof-plane was 

 measured by means of a Dolezalek electrometer. The pro- 

 cess was repeated for various positions of the proof-plane, 

 and in this way the values of a were obtained for various 

 values of 6. The advantages of the form of apparatus em- 

 ployed were two-fold : the surface of the sphere was un- 

 broken except for the narrow ring of insulating material, 

 whilst, since the proof-plane was a fixture, any irregularity 

 of density caused by this discontinuity or by any imperfection 

 in the setting of the proof-plane entered equally into each 

 reading. 



The general arrangement of the apparatus is shown in 

 fig. 3. The screen F consisted of a wooden frame covered 

 with brown paper, which in its turn was covered with silvered 

 paper in order to form a conducting surface. The whole was 

 suspended near the ceiling by means of pulleys and silk cords. 

 A wire G from the screen passed through the wall into an 

 adjoining room, where it was connected to a Wimshurst 

 machine W through a key which made contact with either 

 prime conductor at will. The difference of potential between 

 the screen and the earth was measured by means of a Kelvin 

 Electrostatic Voltmeter V. The screen could be immediately 

 brought to earth potential by means of the discharging-key H. 

 A large Ley den jar, not shown in the figure, was attached to 

 the wire G in order to increase the capacity of the screen and 

 so to render less rapid the fall o£ potential due to leakage. 

 The wire E from the proof-plane of the needle A was carried 



