OF ELECTROSTATIC WITH ELECTROMAGNETIC FORCE. 
647 
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 of a torsion head, graduated, and pro- 
vided with a tangent screw for small angular adjustments. The torsion head was sup- 
ported by a hollow pillar, the base of which was clamped to the lid of the instrument 
so as to admit of small adjustments in every direction. 
The fixed disk and coil were mounted on a slide worked by a micrometer-screw, 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, 4T3 inches diameter, was free to -move, leaving an interval of -065 of 
an inch between the disk and the aperture. A glass scale with divisions of ywo 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 of 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 micrometer-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 the motion 
was interrupted. A very small motion was then sufficient to bring the whole circum- 
ference 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. 
This disk was then brought to the position of first contact, and the microscope was 
adjusted so that a known division of the glass scale was bisected by the cross wires. A 
small piece of silvered glass was fastened to the outside of the guard-ring, and another 
to the back of the suspended disk ; and these were adjusted so as to be in one plane, and 
to give a continuous image of reflected objects when the disks were in contact and the 
surface of the suspended disk was therefore in the plane of the surface of the guard-ring. 
The fixed disk was then screwed back, and the torsion-balance was adjusted so that the 
suspended disk when in equilibrium was in precisely the same position as before. 
This was tested by observing the coincidence of the zero division of the glass scale with 
the cross wires of the microscope, and by examining the reflections from the two pieces 
of silvered glass. The torsion-balance could be moved bodily in any horizontal direction 
by adjusting the base of the pillar; it could be raised or lowered by a winch, and it 
could be turned about any horizonal axis by sliding weights, and round the vertical 
axis by a tangent screw of the torsion head. In this way the position of equilibrium 
