between Plates for small Distances. 149 



parallel plates to be brought into contact throughout the 

 entire extent of the surfaces. This is probably due to the 

 existence of a gaseous film surrounding the surfaces. In 

 exerting a pressure sufficient to squeeze out this last vestige 

 of the air, strains are introduced which destroy the parallelism 

 of the plates. Apart from this the electrical conditions 

 render it advisable that one surface be convex. 



Description. — In the experiment here described, one of the 

 surfaces is convex, the other plane. Jn the accompanying 

 diagram, let " D " be the fixed body, rigidly attached to a 

 firm support A. This fixed body consisted of a steel bicycle 

 ball, heavily nickel-plated, and 2*52 cm. in diameter. This 

 was carefully insulated from the support, and connected 

 with the binding-post " B," to which the necessary electrical 

 connexions were made. The complication due to the sphe- 

 ricity of this surface is not so great as might be supposed, 

 since the distances to be measured are of the order of a 

 wave-length of light, whence a radius of curvature of 1*26 cm. 

 is comparatively large. 



The surface " D' " consists of a brass disk ground plane, 

 then heavily nickel-plated and polished. This was mounted 

 on a movable carriage, " C," which bears the mirror, " M." 

 The mirror M constitutes one arm of the interferometer. 

 The surface D' could thus be brought into contact with D by 

 shifting the carriage. The point of contact was determined 

 by means of a sensitive galvanometer with a small E.M.F. in 

 •circuit. The carriage having been pushed back until the 

 point of contact was reached, the motion of the carriage was 

 reversed ; the distance through which it was moved could be 

 ascertained by counting the number of fringes which passed 

 during the displacement of the carriage. 



The second condition imposed requires the measurement 

 of differences of potential. In the experiments of Lord Kelvin 

 and Liebig an absolute electrometer was used. The smallest 

 difference in potential was approximately 2 C.G.S. units 

 E.M.F. in the electrostatic system. 



In this experiment the measurements of potential were 

 made in the electromagnetic system, the readings being- 

 expressed in volts. The values obtained can therefore be 

 transformed into electrostatic C.G.S. units by dividing by 

 300. A standard Weston voltmeter reading directly to 

 volts was utilized in measuring the differences in potential. 

 The instrument used was compared with a standard volt- 

 meter recently calibrated by the Weston Company and kept 

 in the laboratory for standardizing purposes. 



Source of E.M.F. — The E.M.F. used was supplied by a 



