I 8 2 ELECTROMETRY. 



gauge should be sensitive enough to indicate differences of potential 

 without appreciable effect on the electrometer itself. 



The movable plate is square, and turns about an axis formed 

 of a fine stretched platinum wire. In the position of equilibrium 

 of the wire, the plane of the plate is not that of the ring. When 

 the two planes coincide, electrical attraction counterbalances the 

 torsion of the wire. 



The method of sighting, which enables us to determine the 

 adjustment of the mark, differs slightly from that described above. 

 The small plate of aluminum extends as a tongue h of the same 

 metal a little larger than the radius of the box J, and the end 

 of which is forked. Across this is stretched the wire which serves 

 as sight. This end being outside the box, is under the action 

 of external electrical masses ; and from the length of the lever it 



Fig. 152. 



forms, serious errors might arise. Sir W. Thomson avoids them 

 by protecting the end by metal wires connected with the box, and 

 so arranged as not to intercept the sight. The cross wire should 

 be brought exactly between two black marks on white paper, and 

 at a less distance than the thickness of the wire. The wire and 

 the two points are viewed by means of a lens. In order to avoid 

 errors of parallax as much as possible, Sir W. Thomson uses a 

 small plano-convex lens, the plane surface being turned towards 

 the eye. The wire is just within the focal distance, and the eye 

 should be at a considerable distance on the other side at least 

 20 centimetres. In these conditions the field is reduced to a 

 minimum. When the line of sight coincides with the optical axis, 

 the wire appears as a right line thinner at the extremities. If it 

 is a little out, the image of the wire is curved in one direction 

 or the other. 



