Constants of a Rectangular Galvanometer. 



and 



cd 



c d Q (l + ctt) l + "t 



= G (1— Xt) y 



whence /3= — «. 



§ 17. To test the performance of the galvanometer, a 

 determination of H was made by its aid, and the value so 

 obtained was compared with that given by the Helmholtz 

 coils of a cylindrical galvanometer of the type described in 

 Gray's 'Absolute Measurements/ vol. ii. p. 353. All ne- 

 cessary precautions were taken, the constants of the two 

 galvanometers being determined by measurements which in 

 all cases were referred to a standard metre scale, and the 

 current balances used being carefully compared with each 

 other. 



As the measurements involved present no particular novelty, 

 the results only are given, 



I. Helmholtz Galvanometer, cylindrical coils. 

 Constant of galvanometer = 1*85705. 

 Values of H: — 



•17203, -17212, -17208, -17217, -17201, -17181, -17176, 

 •17183, -17165, -17171, -17141, '17145, -17147, -17121, 

 •17157, -17156, -17148, -17146, -17139. 



Mean H = -17164 c.G.s. units. 



II. Rectangular Galvanometer. 

 Galvanometer constant = '231085. 

 Values of H: — 



•17205, -17129, -17168, -.17144, -17147, -17097, -17180, 

 •17207, -17223, -17239, -17116, -17187, -17215, -17164, 

 •17170, -17234. 



Mean H = -17177 c.G.s. units. 



The agreement between the two results represents about 

 the limit of accuracy in reading the current balances. 

 Moreover, no account has been taken of the diurnal varia- 

 tions in H, which would sensibly affect the fifth decimal 

 place. 



