306 Prof. H. A. Kowland on the Ratio of the Electrostatic 



It had two coils each of about 70 layers of 80 turns each, of 

 No. 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 

 galvanometer described in the American Journal of Science, 

 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 

 Hence we have 



. 10*4141 



G = 19087. 



Electrodynamometer. — This was almost an exact copy of 

 the instrument described in Maxwell's Treatise of 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. 



Q _c\/K ,- 



» — — - — vsma, 

 t 



where K is the moment of inertia of the suspended coil, t its 

 time of vibration, a. the reading of the head, and c a constant 

 depending on the number of coils and their form. 



Large Coils. 



Total number of windings 

 Depth of groove 



Width of groove 

 Mean radius of coils . 

 Mean distance apart of coils 



240 



•84 centim. 



•86 „ 

 13*741 „ 

 13*786 „ 



