Electrostatic and Electromagnetic Units of Electricity. 305 



working parts are enclosed in sheet brass to prevent exterior 

 action. 



As the balance cannot be in equilibrium by combined 

 weights and electrostatic forces, it was found best to limit 

 its swing to a -^ millim, on each side of its normal 

 position. The mean of two readings of the distance, one to 

 make the hair jump up and the other down, constituted one 

 reading of the instrument. 



The adjustments of the plates parallel to each other, and 

 of the movable plate in the plane of the guard-ring, could be 

 made to almost -^ millim. 



The formula for the difference of potential of the two 

 plates is 



A ' 



where d is the distance of the plates, wg the absolute force 

 on the movable plate, and A its corrected area. According 

 to Maxwell 



A=i,r{R2 + E'2-(R' 2 -RS)^}, 



where R and R' are the radii of the disk and the opening for 

 it, and a = *221 (R' — R). The last correction is only about 

 1 in 500, and hence we have finally 



V = 17,22WV4 1+ -^T~ • 



Standard Condenser. — This very accurate instrument was 

 made from my designs by Mr. Granow, then of New 

 York, and consisted of one hollow ball, very accurately turned 

 and nickel-plated, in which two balls of different sizes could 

 be hung by a silk cord. The balls could be very accurately 

 adjusted in the centre of the hollow one. Contact was made 

 by two wires about t Jq inch diameter, one of which was pro- 

 truded through the outer ball until it touched the inner one ; 

 by a suitable mechanism it was then withdrawn and the second 

 one introduced at another place to effect the discharge. This 

 could be effected five times every second. The diameter of 

 the balls has been accurately determined by weighing in 

 water, and the electrostatic capacities found to be 

 50-069 and 29 556 C.G.S. units. 



A further description is given in Mr. Rosa's paper. 



Galvanometer for Electrical Discharges, — This was very 

 carefully insulated by paper, and then put in hot wax in a 

 vacuum to extract the moisture and fill the spaces with wax. 



