ELECTROMETERS. 



235 



of a canoe than like a needle (Fig. 5) ; but, you know, in 

 electricity and magnetism the name ( needle is commonly 

 applied to bodies that in shape and size are very unlike 

 that from which the name is taken. The needle is borne 

 horizontally on a stiff vertical platinum wire, which passes 

 through its centre upward and downward. The stiff platinum 

 wire is attached to a small cross-bar, which is carried by a 

 bifilar suspension. [See Fig. 6, where the upper end of the 

 stiff platinum wire is seen coming up through the " guard 

 tube " and having the " mirror " attached. The bifilar 

 fibres of the suspension are seen coming up to two pins, 

 c and d.] To the lower end of the stiff platinum wire a very 

 fine platinum wire is attached. This carries a small plati- 

 num weight, which hangs down and dips into the sulphuric 



Fio. 5. 



acid at the bottom of the jar; and the needle is by this 

 means kept in connection with the interior coating of the 

 charged Leyden jar. 



I have got here the suspension plate of an electrometer 

 to show you (Fig. 6). It is supported and insulated by 

 a glass pillar, which rises from the main cover, and it is 

 covered by the lantern. On the face of it are five pins, 

 (a) (6) (c) (d) and (h), which can be turned by means of a 

 square pointed key. The threads of the bifilar are wound up 

 on the pins (c) (d), and (c) and (d) are capable of being turned 

 so as to adjust the lengths of the fibres to be equal. The 

 pins (c) and (d) are carried on springs (e) and (/), which are 

 screwed to the face of the suspension plate by screws shown 

 about one -third of way from the bottom of that plate. The 

 pin marked (h) is a conical plug, which passes in between 



