ELECTROMETERS. 



171 



then parallel to the projecting arm above. But in any 

 other position, each fiber is at an angle with its vertical 

 position, and the needle slightly elevated : conse- 

 quently the force of gravity tends constantly to turn 

 the needle, without friction, back to its fixed position. 

 This mode of suspension is termed bifilar. 



A platinum weight, suspended in the sulphuric acid 

 by a fine platinum wire, from the lower end of the stiff 

 wire below the needle, keeps 

 the needle in position, and in 

 contact with the inner coat- 

 ing. 



The wire, above and below 

 the needle, is inclosed in fixed 

 guard tubes ; the lower one 

 shown at w : which screen it 

 from external electric influ- 

 ence; and furnish a COimeC- Fig. 56-Quadrants and Needle. 



tion, by which the charge is given to the inner coating. 



The needle is inclosed within four brass quadrants : 

 which, if joined, would form a circular box. They are 

 separated from each other, and from the needle, as 

 shown in Fig. 56 : and opposite pairs, A and A', B and 

 B f , are connected by fine wires ; and all supported at 

 the same level ; and insulated, by glass rods attached 

 to the cover. 



Three of them are permanently attached, but the 

 fourth can be moved in and out horizontally ; guides, 

 and a spring and counteracting screw, being arranged 

 to keep it in position, and regulate its movement. 



Above the needle, and attached to its supporting 

 wire, is a small concave mirror t; by which a ray of 

 light is reflected on a scale, placed in front of it, at a 



