268 HENRY A. KOWLAND 



Galvanometer for Electrical Discharges. This was very carefully m- 

 sulated 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 

 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 galvano- 

 meter. 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 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 instru- 

 ment 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 



- 



T ysin a 



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. 



