6 Dr. Fletcher on the Determination of the B.A. Unit 



measurement and 1833-67 by comparison. The mean, giving 

 the second value twice the weight of the first, is 1833*19. 



Hence the constant term = 10996 + 10 7 . 



G' has recently been remeasured and found to be 1832*53. 

 My final result is corrected to this value. 



H was measured in the following manner : — The circle 

 bearing the fixed coil carried four smaller wires, which could 

 be connected with the battery and an electrodynamometer of 

 the form described in Maxwell's treatise. These four wires 

 with the needle formed a tangent-galvanometer, the other 

 coils being open. Eight pairs of simultaneous readings of 

 the galvanometer and electrodynamometer were taken, com- 

 prising all possible combinations of signs of the currents in 

 the galvanometer and the two electrodynamometer-coils. I 

 am greatly indebted to Professor S. H. Freeman, then Fellow 

 of the University, for assistance in these readings. 



The expression for H is 



47r 2 nC^TA~6 77 ^ 2 ) 



H = 



-•/sin a 



V Ttain-6 ' 



where C is a function of the dimensions of the electrodyna- 

 mometer-coils, I the moment of inertia of the suspended coil, 

 n the number of turns of wire in the galvanometer, V the 

 mean length of one turn, V the mean distance of their planes 

 from the needle, T the time of vibration of the small coil, 

 a and <j> the mean deflections of electrodynamometer and gal- 

 vanometer. ^ 



C was known from measurements during the construction 

 of the instrument, and I had been determined by observing 

 times of vibration with and without the addition to the sus- 

 pended coil of bars of known moment of inertia. These values 

 of and I had been verified by Dr. E. H. Hall and myself, in 

 connection with a previous research, by comparing the values 

 of H obtained by this method and by the magnetic method, 

 the arrangement of the experiment being such as to make the 

 two results obtain for the same point and time. The value of 

 C^I is 0*18567. 



The measurements of V and V were made in the same 

 manner as those of I and b. 



The results are / ; = 263*91 centim., 6 / = 2'07centim., for most 

 experiments. A correction was applied when V varied. Hence 



4^1(1-6^ =-11069. 



Each of the angles, a and <£, is the mean of eight readings 



