B I FILAR BALANCE. 



61 



Snow Harris,* but really introduced into practice by the researches 

 of Gauss and Weber, f 



In the bifilar balance the movable system is suspended to two 

 adjacent wires, which are usually fixed at the same height above and 

 below. When the apparatus is displaced from its position of equi- 

 librium, the weight of the system gives rise to a couple which tends 

 to restore it to its original direction. 



We shall first assume that the two threads are equal, the two 

 points of attachment B and B' (Fig. 134) at the same height, and 



Fig. 134- 



that the centre of gravity of the suspended body is in the vertical line 

 which passes through the middle O of the distance of the two points 

 of attachment A and A' of the lower ends of these wires. In these 

 conditions, equilibrium exists when the four points of attachment are 

 in the same vertical plane, the line AA' being parallel to the line BB', 

 and that therefore the two wires are symmetrical in respect of the 

 vertical which passes through the centre of gravity of the system. 



Let AA' = 2*, BB' = 2 , AB = /. 



If the diameter AA' be turned through the angle B about the 

 right line OO', each of the wires is inclined to the vertical while still 



* SNOW HARRIS. Phil. Trans., 1836, p. 417. 



t GAUSS et WEBER. Resultate aus den Beob. des Magn. Vereins. 



1837. 



