1881.] Determination of the Ohm in Absolute Measure. 107 



small error of level may be represented by writing G (1 + tan I . ft) for 

 Gin (3) or (4). 



The next step is to express GK in terms of the measurements of the 

 coil. In order that there may be a passage for the suspending fibre 

 and its enveloping tube, it is necessary that the coil be double, or if 

 we prefer so to express it, that there be a gap in the middle. If 



a = mean radius of each coil, 



n— whole number of windings, 



b = axial dimension of section of each coil, 



c= radial dimension of section of each coil, 



b' = distance of mean plane of each coil from the axis of motion, 

 a=angle subtended at centre by radius of each coil, so that cot«= 

 b'ja, 



The correction due to the finiteness of b and c is in practice extremely 

 small, but the factor sin 3 a. must be determined with full accuracy. 



In order to arrive at the value of MK/GH, which occurs in (3), we 

 observe that the approximate value of K/G is 2 sin 3 a / a 3 ; so that 



