234 Duncan, Wilkes and Hutchmson — On the 



A happens to be undesirably large after beginning the experi- 

 ment, B is made to give a small deflection, and A' made as 

 nearly as may be the same as A. We have in this case to 

 apply the mean of two corrections to E a , one from A and the 

 other from A'. 



In each arrangement, as A, the current is reversed four 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 25 galvanometer readings, and 

 occupies five minutes. The set B is begun as quickly as 

 possible after A. The chronograph record is started by drop- 

 ping 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 resistance carrying the main 

 current is constantly stirred and the others frequently. After 

 A', the temperature of the cylinder and disc 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 coefficients of mutual induction for the two pairs of coils 

 as used by Professor Rowland are : — 



Coils 1 + 4, M= 60292-5 

 " 2 + 3, M = 102030-2 

 Diameter disc =42-1334 at 17°. 



Before beginning these experiments, the disc was slightly 

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

 sured by two observers, and was found to be 



43-1201 at 17° C. 



The formulae expressing the effect on M of small changes in 

 the quantities entering in its expression are, 



dM dA da „ db 



for ] +4, -- - =-015 -j- -f 1-912 — —"927 -=- 



, M A a o 



and e dM dA da H db 



for 2 + 3, ^n- =-0-95 -r- +2-12 0-17 -j-, 



M A a o 



where A = mean radius of the coil, 

 a= radius disc, 

 #=distance of mean planes of disc and coil. 



The corrections, calculated by these formulae, due to the 

 change in " a " give 



