WITH ELECTROMAGNETIC FORCE. 131 



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 horizontal axis by sliding weights, and round the vertical 

 axis by a tangent screw of the torsion head. In this way the position of 

 equilibrium of the suspended disk could be made to coincide with the plane 

 of the guard-ring to the thousandth of an inch; and the adjustment when 

 made continued very good from day to day, soft copper wire, stretched straight, 

 not having the tendency to untwist gradually which I have observed in steel 

 wire. The weight of the torsion piece was about 1 Ib. 3 oz., and the time of 

 a double oscillation about fourteen seconds. The oscillations of the suspended 

 disk, when near its sighted position, were found to subside very rapidly, the 

 energy of the motion being expended in pumping the air through the narrow 

 aperture between the guard-plate and the suspended disk. 



The electrical arrangements were as follows : 



One electrode of Mr Gassiot's great battery was connected with a key. 

 When the key was pressed connexion was made to the fixed disk, and thence, 

 through Mr Willoughby Smith's resistance-coils, to a point where the current 

 was divided between the principal coil of the galvanometer and a shunt, S, 

 consisting of Mr Jenkin's resistance- coils. These partial currents reunited at a 

 point where they were put in connexion with the other electrode of the battery, 

 with the case of the instrument, and with the earth. 



Another battery was employed to send a current through the coils. One 

 electrode of this battery was connected with a second contact piece of the 



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