ROSA, DORSET AND MILLER: INTERNATIONAL WIl'KKF. 215 



ments were made experimentally. Currents were then passed through 



the two coils in opposite sense and adjusted until the resultanl magnetic 

 field at the center was zero. This was indicated by a magnetometer 

 needle, 2 mm. long. The ratio of the two currents which balance each 

 other in this manner is equal to the inverse ratio of the "galvanometer 

 constants" of the two coils, excepting for a small correction on account 

 of the length of the needle. The ratio of the "galvanometer constants" 

 gives the ratio of the two mean radii when the numbers of turns and the 

 sectional dimensions of the coils are known. 



The ratio of the current was measured in two ways which have been 

 called the "potentiometer" and "shunt" methods, respectively. Tic 

 first method consisted in a simultaneous measurement of the two cur- 

 rents by means of two potentiometers. The second method was em- 

 ployed in cases where, as designed, the coils had nearly the same gal- 

 vanometer constants. In this method the two coils are joined in series 

 in such a manner that the current flows through them in opposite sense. 

 As used in this work both methods permitted an accuracy of about two 

 parts in a million. The latter method, however, is somewhat the more 

 accurate, considerably more rapid, and requires but one observer. In 

 the table is given the results of a series of measurements of the ratio 



November 28 



November 29. 



December 2. 



1.004596 

 94 

 95 

 94 

 93 

 89 

 94 

 93 

 92 

 95 

 96 

 92 

 92 

 91 

 91 

 94 

 93 

 92 



3 



1 

 2 



1 

 

 4 

 1 

 

 1 

 2 



o 

 O 



1 



1 



2 

 2 

 1 

 

 1 



Mean. 



1.004593 



1 I 



