172 ELEMENTS OF STATIC ELECTRICITY. 



distance of about 36 inches. This scale is shown in 

 Fig. 57. Behind it is a lamp, the light from which 

 comes through a vertical slit in a screen : above which 

 is a horizontal screen, which cuts off the direct rays 

 from below ; while the angle of reflection brings the 

 ray from the mirror directly on the scale, where it 

 appears as a small spot of light. Another screen, 

 placed at an angle, 

 cuts off the direct 

 rays from above. 



As the mirror turns 

 with the needle, the 

 reflected ray becomes 

 a long pointer ; mov- 

 ing without friction : 

 by which the slight- 

 est movement of the 

 needle is indicated on 



Fig. 57 Scale, Lamp, and Screen. 

 In the instruments 



first constructed, the needle was suspended by a single 

 fiber of silk ; and a small magnet attached to the back 

 of the mirror : which, by the attraction between it and 

 a large magnet, placed outside the jar, as shown in Fig. 

 55, controlled and limited the movements of the needle ; 

 the attraction of the magnets tending constantly to 

 bring it back to its fixed position, where the spot of 

 light rests on the zero of the scale. But the bifilar sus- 

 pension is now preferred ; rendering the use of magnets 

 unnecessary. 



At I and m. Fig. 55, are seen the chief electrodes ; 

 used to connect the opposite pairs of quadrants with 

 bodies whose potential is to be tested : and at p is the 



