Ox THE ABSOLUTE UNIT OF ELECTRICAL RESISTANCE 167 



this reason the constant was also found by comparison with another 

 coil. The following were the dimensions: 



Mean radius 4-3212 cm. 

 R - 5-6212 r = 3-0212 



X= 3-475565 x= -935565 



R r = 2-6000 X x = 2-54000 



^=1790- 

 whence 



F= 1832-25 1-70&'& (0) - 4-50i 4 & (0) + -90 6 () 6 (0) - &c. 

 Taking the mean dimensions of the two needles, we have 



1 = 1-23, w = -52, w = |, cos 6' = -748. 

 Q t (0') = + 339 , Q t (6'} = - -354 , Q 6 (a') = - -275 . 

 .-. G = 1832-25 -083 + -071 - -002 + &c. = 1832-24. 



The coil with which this galvanometer was compared was the large 

 coil of an electro-dynamometer similar to that described in Maxwell's 

 'Electricity/ Art. 725, but smaller. The coil was on Helmholtz's 

 principle with a diameter of 27-5 cm., and was very accurately wound 

 on the brass cylinder. There was a total of 240 windings in the coil. 

 The constant of this coil was 78-371 by calculation. 



To eliminate the difference of intensity of the earth's magnetism, an 

 observation was first made and then the positions of the instruments 

 were changed so that each occupied exactly the position of the other: 

 the square root of the product of the two results was the true result 

 free from error. 



The coils of the galvanometer could be separated so that an outer 

 and inner pair could be used together. By comparing these parts 

 separately and adding the constants together we find G. Hence two 

 comparisons are possible, one with the coils together and the other with 

 them separate. The results were for the ratio of the constants 



23-3931 and 23-4008, 

 which give 



G = 1833-37 and 1833-98. 

 The mean result is 



1833-67 -09, 



and this includes seven determinations with two reversals of instru- 

 ments. This result is one part in thirteen hundred greater than found 

 by direct calculation, which is to be accounted for by the small size of 

 the galvanometer coils and the consequent difficulty of their accurate 

 measurement. As comparison with the electro-dynamometer has such 



