ELECTROMETERS. 157 



center, a glass tube D D, to each end of which is fitted 

 a brass collar. An enlarged section of the upper end 

 of this tube and its attachments, representing what is 

 known as the torsion head, is shown separately ; in 

 which it will be noticed, that the brass collar a has 

 fitted to it a cap b with a projecting rim ; on the 

 upper surface of which is a graduated scale, of 360 

 equal divisions. This cap is capable of being turned 

 horizontally, so as to bring the several divisions of the 

 scale under a pointer <?, attached to a. 



In the center of b is a close fitting brass rod c?, 

 with a broad head by which it can be turned, when b 

 is held firmly ; or the rod may be allowed to turn with 

 b. Attached to this rod is a fine wire, which sustains, 

 at its lower extremity, a horizontal shellac rod /, 

 carrying at one end a small gilt ball g. Opposite this 

 ball, on the cylinder A A, is & graduated scale a a, 

 having 360 divisions, to correspond to those of the 

 upper scale. Opposite the zero of this scale is a gilt 

 ball g f , of the same size as the other gilt ball, and sup- 

 ported by a shellac rod /", by which the ball can be 

 introduced through an opening in the top of A A. 



The instrument is supported on a base, having level- 

 ing screws ; and the air, in the interior, kept dry with 

 chloride of calcium. 



To use this instrument, the cap b is turned till the 

 zero of the upper scale is brought under the pointer c. 

 The rod d is then turned till the movable ball g just 

 touches the fixed ball </, without torsion of the wire. 

 The zeros of the two scales will then be practically in 

 the same vertical plane. 



The fixed ball cf is then taken out, electrified, and 

 replaced as before, in contact with the movable ball g. 



