Prof. Thomson on the Measurement of Electromotive Force. 317* 



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 when touching the metal below, was always deter- 

 mined electrically by making a succession of sparks pass, and ap- 

 proaching the moveable disc gradually by the screw until all appear- 

 ance 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. 



Table I. — December 13, 1859. Measurements by absolute electro- 

 meter of maximum electrostatic forces * across a stratum of air 

 of different thicknesses. 

 Area of each plate of absolute electrometer=*187 of a square foot. 

 Distance between plates of absolute electrometer =^ of a foot. 



* See § 3 below. 



