SOME SPECIAL MUTUAL CHANGES 



75 



gold leaves are replaced by a thin flat strip of aluminium, 

 which is suspended by two silk fibres so as to hang hori- 

 zontally within four hollow horizontal metal quadrants. The 

 opposite pairs of quadrants are united by a wire, but they are 

 otherwise quite separate from one another, and supported on 

 glass pillars. The whole is covered by a glass case, and also 

 by a metal framework which places the whole of the space 

 around the electrometer in more perfect connection with the 

 earth. Two pairs of quadrants may, by means of connecting 

 metal rods, be placed in electric communication with two 

 bodies which have been electrified. These pairs of quadrants 

 now electrically represent, in a more investigable form, the 

 bodies with which they have been placed in contact. Or 

 rather, we add to our system another symmetrical system, 

 which partakes of its electrical condition and exhibits it in a 

 convenient manner. The aluminium vane may, if necessary, 

 be electrified by communication with an external body, 

 through the medium of hydrogen sulphate (contained in a 

 vessel underneath) and platinum wires. These together con- 

 nect the moveable vane 

 with a metal rod, which 

 leads out wards and turns 

 on a pivot. The vane is 

 made to hang so that it 

 is midway between the 

 two pairs of quadrants 

 when they are' not elec- 

 trified. It moves to the 

 one side or the other 

 under different electric 

 stresses ; and the deflec- 

 tions are marked by the 

 movement on a scale of 

 a spot of light, which 

 comes from a lamp, and is reflected from a small mirror 

 attached to the plate. The essential structure of this instru- 

 ment is here show in diagram (fig. 24). 



By the use of the quadrant electrometer we may perceive 



---M IRROR. 



SECTION. 



PLAN. 



Fig. 24. 



