514 pf*"***- CONSIDERATIONS 



13. Hi FILAR SUSPENSION. 



elastic force of torsion of a wire, though much more regular than the 

 force of friction, is subject to alterations arising from hitherto unknown causes, 

 but probably depending on facts in the previous history of the wire, such as 

 iu baring been subjected to twists and other strains before it was hung up. 

 it is sometimes better to employ another mode of suspension, in which 



force of restitution depends principally on the weight of the suspended parts. 



The body is suspended by two wires or fibres, which are close together 

 and nearly vertical, and are so connected by a pulley that their tensions are 

 equal. The body is in equilibrium when the two fibres are in the same plane. 

 When the body is turned about a vertical axis, the tension of the fibres produces 

 a force tending to turn the body back towards its position of equilibrium ; and 

 this force is very regular in its action, and may be accurately determined by 

 proper experiments. 



This arrangement, which is called the Bifilar suspension, was invented by 

 Gauss and Weber for their magnetic apparatus. It was afterwards used by 

 Baily in his experiments on the attraction of balls. 



14. METHODS OF READING. 



The observed position of the indicating part of an instrument is recorded 

 as the "Reading." To ascertain the position of the indicating part of the 

 instrument various methods have been adopted. The commonest method is to 

 make the indicating part in the form of a light needle, the point of which 

 moves near a graduated circle. The position of the needle is estimated by 

 observing the position of its point with respect to the divisions of the scale. 

 By giving the needle two points at opposite extremities of a diameter, and 

 observing the position of both points, we may eliminate the errors arising from 

 the want of coincidence between the centre of the graduated circle and the 

 axis of motion of the needle. 



This is the method adopted in ordinary magnetic compasses. As it is 



necessary for freedom of motion that the point of the needle should not be in 



ctual contact with the graduated limb, the reading will be affected by any 



change in the position of the eye of the observer. The error thus introduced 



