128 Lord Rayleigh and Dr. A. Schuster. [May 5, 



To obtain the circumference of the axis of the mean wind- 

 ing we have to subtract tt x thickness of wire ..../..= "431 



Hence the final value of the mean circumference /3= 99 '204 



Or for the mean radius a= 15 '789 



The grooves of the coils and their distance was also 

 measured, and it was found that — 



b = axial dimension of coil = 1 '833 



b' = distance of mean plane from axis of motion. . . = 1 '918 



All measurements are given in centimetres. 



We calculate — 



a=angle subtended at axis by mean radius =cot _1 — . . . = 83° 04/ 



a 



And log sin 3 « = 1 -990458 



The principal term in the expansion of GK is 7m 2 /3 sin 3 a,= 29,869,300 

 To this is to be added a small correction given on p. 107 — 100 



The final value of GK being 29,869,200 



Applying the corrections for level and torsion to GK as explained, 

 and writing (j&ii for the value so corrected, we find, 



<Mt=29,887,6Q0, 



The Observations. 



The observations consisted of two parts : the comparison of the 

 resistance of the rotating copper coil with that of a standard German 

 silver coil, and the observation of the deflections during the spinnings. 

 The comparison of resistance was made by a resistance balance devised 

 by Mr. Fleming,* to whom we are indebted for advice and assistance 

 in all questions concerning the comparison of resistance. In this 

 balance, which only differs by a more convenient arrangement from an 

 ordinary Wheatstone's bridge, Professor Carey Foster's method of 

 comparing resistance is used. The method consists in interchanging 

 the resistance in the two arms of the balance which contain bhe 

 graduated wire, and thus finding the difference between these two 

 resistances in terms of that of a certain length of the bridge wire. 

 The balance was placed on a table near the rotating coil, and could be 

 electrically connected with it by means of two stout copper rods. 

 The German silver coil which served as the standard of comparison 

 was prepared so as to have a resistance nearly equal to that of the 

 copper coil, that is about 4" 6 ohms. Any error due to thermo-electric 

 currents, which have sometimes been found to be generated at the 



* " Phil. Mag.," ix, p. 109,^1880. 



