236 LECTURES TO SCIENCE TEACHERS. 



the springs and screws into the plate behind, and by turn- 

 ing it in or out the conical plug presses the springs (e) and 

 (/) apart, or allows them to approach. Thus the upper 

 points of the bifilar suspension are separated or brought 

 nearer together, and the sensibility of the instrument is 

 diminished or increased. The pins (a) (b) are screwed into 

 the springs (e) (/), and press against the plate behind. 



FIG. 6. 



When one or other of them is turned it brings forward 

 the neighbouring point of suspension, or allows it to recede. 

 Lastly, as to this part, the platinum wire carrying the 

 needle has a small circular mirror attached to it (see Fig. 

 6), and all the movable parts are carefully guarded from 

 external influence ; the platinum wire, both above and 

 below the needle, by a metal " guard tube " through which 

 it passes, and the mirror by a little cylindrical hood project- 

 ing some distance from the suspension plate. 



