to the Electromagnetic Units of Electricity. 307 



Suspended Coils, 



Total number of windings . . 126 



Depth of groove '41 centim. 



Width of groove -38 „ 



Mean radius 2' 760 „ 



Mean distance apart .... 2' 707 „ 



These data give, by Maxwell's formulae, 



c = '006457. 



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

 tangent galvanometer with a circle 80 centim. diameter, and 

 the earth's magnetism was determined many times by passing 

 the current from the electrodynamometer through this instru- 

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

 In this way the following values were found : — 



Magnetic Method. Electrical Method. 

 Dec. 16th, 1879 -19921 -19934 



Jan. 3rd „ '19940 '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 

 necessary 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 centre and placing weights at different distances along 

 it. In this way was found 



K= 826-6 C.G.S. units. 



The use of the electrodynamometer in the experiment was 

 to determine the horizontal intensity of the earth's magnetism 

 at any instant in the position of the ballistic galvanometer. 

 This method was necessary on account of the rapid changes 



