122, 123] 



The Quadrant Electrometer 



ior 



so that 



It is now only necessary to measure 7^ /i, 2 , the distance through which 

 the lower plate is moved forward, and this can be determined with great 

 accuracy, as it depends solely on the motion of the micrometer screw. 



II. The Quadrant Electrometer. 



123. Measurement of Potential Difference. This instrument is more 

 delicate than the disc electrometer just described, but enables us only to- 

 compare two potentials, or potential differ- 

 ences; we cannot measure a single potential 

 in terms of known units. 



The principal part of the instrument 

 consists of a metal cylinder of height small 

 compared with its radius, divided into four 

 quadrants A, S, C, D by two diameters at 

 right angles. These quadrants are insulated 

 separately, and then opposite quadrants 

 are connected in pairs, two by wires joined 

 to a point E and two by wires joined to 

 some other point F. 



The inside of the cylinder is hollow and 

 inside this a metal disc or "needle" is free 

 to move, being suspended by a delicate 

 fibre, so that it can rotate without touching 

 the quadrants. Before using the instrument 

 the needle is charged to a high potential, 

 say v, either by means of the fibre, if this 

 is a conductor, or by a small conducting 

 wire hanging from the needle which passes through the bottom of the 

 cylinder. The fibre is adjusted so that when the quadrants are at the same 

 potential the needle rests, as shewn in the figure, in a symmetrical position 

 with respect to the quadrants. In this state either surface of the needle 

 and the opposite faces of the quadrants may be regarded as forming a parallel 

 plate condenser. 



If, however, the potential of the two quadrants joined to E is different 

 from that of the two quadrants joined to F, there is an electrical force 

 tending to drag the needle under that pair of quadrants of which the potential 

 is more nearly equal to v. The needle accordingly moves in this direction 

 until the electric forces are in equilibrium with the torsion of the fibre, and 

 an observation of the angle through which the needle turns will give an 



FIG. 41. 



