292 77. A. Rowland et al. — Ratio of the Electromagnetic 



(VK 

 o — vsiii 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. 



Large coils. 



Total number of windings 240 



Depth of groove '84 cm. 



Width of groove -76 cm. 



Mean radius of coils 13-741 cm. 



Mean distance apart of coils 13*786 cm. 



Suspended coils. 



Total number of windings 126 



Depth of groove -41 cm. 



Width of groove "38 cm. 



Mean radius 2-760 cm. 



Mean distance apart 2 - 707 cm. 



These data give, by Maxwell's formulae, 



C = 0-006457. 

 In order to be sure of this constant, I constructed a large 

 tangent galvanometer with a circle 80 cm diameter and the 

 earth's magnetism was determined many times by passing the 

 current from the electrodynamometer through this instrument 

 and also by means of the ordinary method with magnets. In 

 this way the following values were found. 



Dec. 16th, 1879 



Jan. 3rd, " 



Al ague tic 

 method. 



.. .-. -19921 

 -19940 



Electrical 

 method. 



•19934 

 •19942 



Feb. 25th, " 



-19887 



•19948 



" 28th, " 



'_. -19903 



•19910 



March 1st, " 



.. -19912 



•19928 









Mean -19912 .19933 



which differ only about 1 in 1000 from each other. Hence 



we have for C : 



From calculation from coils '006457 



From tangent galvanometer -006451 



Mean .006454 c.g.s. units. 



The suspension was bifilar and no correction was found neces- 

 sary for the torsion of the wire at the small angles used. 



The method adopted for determining the moment of inertia 

 of the suspended coil was that of passing a tube through its 

 center and placing weights at different distances along it. In 

 this way was found 



