327 



head divided into 40 equal parts of circumference. The readings 

 on the screw-head could be readily taken to tenth parts of a division, 

 that is to say, to ~^ of an inch on the distance to be measured. 

 The point from which the spark would pass in successive trials being 

 somewhat variable and often near the edges of the discs, a thin flat 

 piece of metal, made very slightly convex on its upper surface like an 

 extremely flat watch- glass, was laid on the lower plate. It was then 

 found that the spark always passed between the crown of this con- 

 vex piece of metal and the flat upper plate. The curvature of the 

 former was so small, that the physical circumstances of its own elec- 

 trification near its crown, the opposite electrification of the opposed 

 flat surface in the parts near the crown of the convex, and the electric 

 pressure on or tension in the air between them could not, it was 

 supposed, differ sensibly from those between two plane conducting 

 surfaces at the same distance and maintained at the same difference 

 of potentials. 



The reading of the screw-head corresponding to the position of 

 the moveable disc, was always determined electrically by making a 

 succession of sparks pass, and approaching the moveable disc gra- 

 dually by the screw until all appearance of sparks ceased. Contact 

 was thus produced without any force of pressure between the two 

 bodies capable of sensibly distorting their supports. 



With these arrangements several series of experiments were made, 

 in which the differences of potentials producing sparks across differ- 

 ent thicknesses of air were measured first by the absolute electro- 

 meter, and afterwards by the portable torsion electrometer. The 

 following Tables exhibit the results hitherto obtained. 



