ELECTROMETERS. 



227 



and it also acts as a part of the inside coating of the 

 Leyden jar, like the water in the celebrated experiment 

 that led to the discovery of the Leyden jar. 



The glass bell is enclosed in a metal case which supports 

 it and protects it, and which is furnished with three level- 

 ling screws as feet of the instrument. The case is covered 

 in with a circular plate of glass in a metal rim. At the 

 centre of this glass plate is supported a torsion head, as in 

 Coulomb's torsion balance, having an index protruding out 



FIG. 2. 



so as to move over graduations on the circular metallic 

 ring. 1 



From the torsion head hangs a fine glass fibre, to which 

 is attached the movable part of the electrometer. This 

 consists of a light horizontal needle made of aluminium. 

 To it there is attached a stiff platinum wire, hanging down 



for electrical experiments are found to maintain a charge much better 

 in dry than in damp air. The extraordinary loss observed in damp 

 air is, however, altogether by means of the supports (of glass, shellac, 

 &c.), the surfaces of which receive a conducting film, often invisible, 

 of moisture from the air. 



1 The parts of the instrument were shown in the lecture. They will 

 be easily distinguished in the accompanying diagram. 



Q 2 



