108 Dr. Gr. A. Liebiof on the Electrostatic Force 



a 



door in the side of which weights could be placed upon the 

 scale-pan without disturbing in any way the movable disk. 

 The guard-ring and lower plate of the electrometer measured 

 nearly 35 centim. in diameter, whilst the movable disk had a 

 diameter of almost exactly 10 centim. All of these plates 

 were ground with great accuracy to plane surfaces. The 

 lower movable plate was insulated from the rest of the appa- 

 ratus by means of its support, which was a vulcanite rod 

 sufficiently thick and strong to keep the plate quite rigid. It 

 ended with a metal stem on which was engraved a millimetre- 

 scale (from to 80 millim.), and fitted into a metal socket 

 provided with a vernier divided to lOths of a millim., and a 

 tangent-screw and clamp. Rough adjustments were made by 

 releasing the clamp and moving the plate by its vulcanite 

 support. It was then clamped and set to within l-10th 

 millim. by the tangent-screw, and readings made by estima- 

 tion to l-20th millim. The plate with its attachments was 

 prevented from falling, when released from its socket, by a 

 counterpoise at the end of a lever. The plates between which 

 the sparks were made to pass were of brass, and were ground 

 to spherical segments. Their radius of curvature was 9*76 

 centim., and diameter 4'83 centim. These plates were firmly 

 attached by insulating handles to the frame and carriage, 

 respectively, of a dividing-engine, with which their distance 

 apart was measured. 



During the experiments on coal-gas, hydrogen, &c, the 

 plates were enclosed in an air-tight cylindrical vessel made of 

 glass and wood, with a capacity of about 400 cubic inches, 

 and provided with stuffing-boxes at each end, through which 

 the rods holding the plates passed. The gases, which were 

 dried over calcium chloride, were admitted into the chamber 

 in a steady stream by upward or downward displacement, 

 according to density. The gases were always at ordinary 

 temperatures (18° to 20° C.) and pressures. In the case of 

 C0 2 , coal-gas, and hydrogen, the pressures in the vessel were 

 just sufficient to force the gases through in an almost constant 

 stream. After every series of experiments the plates were 

 polished and all traces made by the sparks obliterated. This 

 could always be accomplished by a few minutes' rubbing on 

 a concave surface (which had been used originally to grind 

 the plates with) covered with paper and dry rouge. The 

 electricity was furnished by a small Hol«z machine, and the 

 wires leading from it to the electrometer and a single Leyden 

 jar (always kept in the circuit) were covered with glass tubes 

 to prevent leakage as far as possible. 



