102 Messrs. Duncan, Wilkes, and Hutchinson on the Value 



times ; it is kept in the same direction for one minute at a 

 time, and five galvanometer-readings at equal intervals of 

 time are taken each minute : this gives, then, twenty-five 

 galvanometer-readings and occupies five minutes. The set B 

 is begun as quickly as possible after A. The chronograph- 

 record is started by dropping the pen on the revolving drum 

 only a few seconds before the first galvanometer-reading, and 

 an effort is made to use that portion of the record beginning 

 exactly with the readings ; the record is stopped at the instant 

 of the last reading by lifting the pen. Temperature-readings 

 are taken before, after, and often during the set. The resist- 

 ance carrying the main current is constantly stirred, and the 

 others frequently. After A! the temperature of the cylinder 

 and disk is noted. 



Variety was given to the different experiments by using 

 different pairs of induction-coils, inner or outer, and by vary- 

 ing the speed and direction of rotation. Sometimes, too, an 

 experiment was repeated with everything the same, except 

 that the resistance c would be made up of different coils. 



The coefficient of mutual induction for the two pairs of coils 

 as used by Prof. Eowland are : — 



Coils 1 + 4, M= 60292-5. 



„ 2 + 3, M = 102030-2. 



Diameter of disk . =43' 1334 at 17°. 



Before beginning these experiments, the disk was slightly 

 turned off in order to smooth the edge ; the diameter was 

 measured by two observers, and was found to be 



43-1201 at 17° 0. 



The formula? expressing the effect on M of small changes in 

 the quantities entering in its expression are, 



for 1 + 4, B= -015 ^ + 1-912 ^ -927 f; 



M A a b 



for 2 + 3, ^f = -95 ^+2-12 £-17 f; 



where A = mean radius of the coil, 

 a = radius of disk, 

 b = distances of mean planes of disk and coil. 



