130 A DIRBCT COMPARISON OF ELECTROSTATIC 



The suspended ooU, besides the repulsion due to the fixed coil, experiences 

 a couple due to the action of terrestrial magnetism. To balance this couple, 

 a coil exactly similar was attached to the other arm of the torsion-balance, 

 and the current in the second coil was made to flow in the opposite direction 

 to that in the first. When the current was made to flow through both coils, 

 no effect of terrestrial magnetism could be observed. 



The torsion-balance consisted of a light brass frame, to which the suspended 

 coils and disks were attached so that the centre of each coil was about eight 

 inches from the vertical axis of suspension. This frame was suspended by a 

 copper wire (No. 20), the upper end of which was attached to the centre <>t 

 a torsion head, graduated, and provided with a tangent screw for small angular 

 adjustments. The torsion head was supported by a hollow pillar, the base <>t 

 which was clamped to the lid of the instrument so as to admit of sniull 

 adjustments in every direction. 



The fixed disk and coil were mounted on a slide worked by a micrometer- 

 ncrew, and were protected by a cylindrical brass box, the front of which, 

 forming the guard-ring, 7 inches in diameter, had a circular aperture 4 '26 inches 

 diameter, within which the suspended disk, 4'13 inches diameter, was free to 

 move, leaving an interval of '065 of an inch between the disk and the aper- 

 ture. A glass scale with divisions of -j-J^ of an inch was attached to the 

 suspended disk on the side which was not electrified, and this was viewed by 

 a microscope attached to the side of the instrument and provided with cross 

 wires at the focus. 



The disk worked by the micrometer was carefully adjusted by the maker, 

 so as to be parallel to the inner surface of the guard-ring, or front face <>t 

 the micrometer-box. This front face of the micrometer-box, when in position 

 in the instrument, was made vertical by means of three adjusting screws. 

 The suspended disk was then pressed against the fixed disk by means of a 

 slight spring, and the fixed disk was gradually moved forward by the micro- 

 meter-screw, while at the same time the graduated scale was observed through 

 the microscope. In this way the graduations on the scale were compared with 

 the readings of the micrometer. This was continued till the large disk came 

 into contact with the guard-ring at one point, when the regularity of tin- 

 motion was interrupted. A very small motion was then sufficient to bring the 

 whole circumference of the disk into contact with the guard-ring, when the 

 motion ceased altogether. This motion was not much more than one-thousandth 

 of an inch. 



