to the Electrostatic Unit of Electricity. 291 



nickel plated, in which two balls of different sizes could be 

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

 justed in the center of the hollow one. Contact was made by 

 two wires about y-j^ 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 by a suitable mechanism it was then withdrawn and the second 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 diameters of 

 the balls have 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. 

 It had two coils, each of about TO layers of 80 turns each of 

 JSTo. 36 silk covered copper wire. They were half again as 

 large as the ordinary coils of a Thomson galvanometer. The 

 two coils were fixed on the two sides of a piece of vulcanite 

 and the needle was surrounded on all sides by a metal box to 

 protect it from the electrostatic action of the coils. A metal 

 cone was attached to view the mirror through. The insulation 

 was perfect with the quickest discharge. 



The constant was determined by comparison with the gal- 

 vanometer described in this Journal, vol. xv, p. 334. The 

 constant then given has recently been slightly altered. The 

 values of its constant are 



By measurement of its coils 1832-24 



By comparison with coils of electrodynamometer.. 1833*67 

 By comparison with single circle 1832*56 



Giving these all equal weights, we have 



1832-82 

 instead of 1833*19 as used before. 



The ratio of the new galvanometer constant to this old one 

 was found by two comparisons to be 



10-4167 

 10*4115 



Mean, 10*4141 

 Hence we have 



G= 19087. 



Electrodynamometer. — This was almost an exact copy of the 

 instrument described in Maxwell's treatise on electricity except 

 on a smaller scale. It was made very accurately of brass and 

 was able to give very good results when carefully used. The 

 strength of current is given by the formula 



