26 L. B. Fletcher— A Determination of the B. A. Unit 



The quantity b, the excentricity of the needle, was estimated 

 by holding the tape horizontally over the top of the circle and 

 reading the positions of the center of the wire and the galvan- 

 ometer' fiber. For most experiments b=l'2 cm . It was fre- 

 quently re-measured and a correction applied when it varied. 

 The method of measurement is not veiy accurate, but an error 

 of 10 per cent in b, which could hardly occur, would only 

 involve an error of 1 part in 3,000 in J. 



G' had been determined by Professor Rowland* by measure- 

 ment during the construction of the coil, and also by com- 

 parison with another coil. The values are 1832*24 by measure- 

 ment 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 re-measured and found to be 1832'53. 

 My final result is corrected to this value. 



H was measured in the following manner : The circle beai> 

 ing 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 galvan- 

 ometer and electro-dynamometer were taken comprising all 

 possible combinations of signs of the currents in the galvan- 

 ometer and the two electro-dynamometer coils. I am greatly 

 indebted to Professor S. H. Freeman, then Fellow of the Uni- 

 versity, for assistance in these readings. 



The expression for His 



H ..4rt.<V i (i-e^) V: 



sin a 



I' T tan <p 



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, b' the mean distance of their planes 

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

 a. and <p 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 C 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 

 * This Journal, xv, 337, 1878. 



