1881.] Determination of the Ohm in Absolute Measure. 125 



of shellac, to prevent any slipping between the magnets and the 

 mirror. The proper suspension of the magnet is a point of some 

 delicacy and importance. As regards the vertical adjustment, the 

 distance of the cube of magnets from the highest and lowest point of 

 the circular frame was measured, and the magnet raised or lowered 

 until the distances became equal. A pointer was next fixed to the 

 frame, reaching very nearly to the centre of the coil. As the coil 

 was rotated, the pointer described a small circle round the axis of 

 revolution, and the position of the magnet could be easily altered until 

 it occupied the centre of the small circle. It is supposed that this 

 adjustment was made to within less than 1 millim., and could, 

 therefore, for all practical purposes, be considered as perfect. 

 The magnetic moment of the magnet was measured in the usual 

 way. Two closely agreeing sets of measurement showed that at 

 a distance of 1 foot it deflected a suspended needle through an 

 angle, the tangent of which was *000298. Hence at the mean dis- 

 tance of the coil (15*85 centims.) the deflection would have been 



MK 



•0021. This number is equal to — — , and will be referred to as tan 



GH 



in the discussion of the calculations. The magnetic moment was 

 determined a few days after the last spinnings had been taken ; but 

 on each day on which experiments were made, the time of vibration 

 of the magnet was determined, and we thus assured ourselves that no 

 appreciable change in the magnetic moment had taken place while 

 the experiments were going on. The time of one complete vibration 

 was 14*6 seconds. 



Adjustment of Scale and Telescope. — The telescope which served to 

 read the angle of deflection rested on a small table to which it could 

 be clamped. In front of the table and below the telescope, the scale 

 could be raised or lowered and fixed when the proper position had 

 been found. It was levelled by deflecting the magnet successively 

 towards both sides, and observing the point of the scale at which the 

 cross wires of the telescope seemed to cut the scale. If in both 

 positions of the mirror the scale was intersected at the same height, it 

 was considered to be sufficiently levelled. It remained to place the 

 scale at right angles to the line joining its centre to the mirror. This 

 was done by measuring the distance of both ends to the mirror by 

 means of a deal rod, with metallic adjustable pointers (presently to be 

 described), and altering the position until these distances were equal. 

 It is supposed that considerable accuracy was thus obtained. A small 

 remaining error would be eliminated by observing deflections on both 

 sides of the zero. To adjust the telescope we had now only to point 

 it to the centre of the mirror, and at the same time to place it in such a 

 position that its optic axis passed vertically over the centre of the 

 scale. By suspending a plumb line from the telescope so as to divide 



