io SCIENTIFIC APPARATUS. 



the extent of the sliding motion small compared with the motion 

 of the indicating part, we may reduce the effect of friction to a 

 very small part of the whole effect. 



This is done in rotating parts by diminishing the size of the axle, 

 and by supporting it on friction- wheels ; and in toothed wheels 

 by keeping the bearings of the teeth as near as possible to the line 

 of centres, or more perfectly by cutting the teeth obliquely, as in 

 Hooke's teeth.* A compass needle is balanced on a fine point, 

 and the extent of the bearing is so small that a very small 

 force applied to either end of the needle is sufficient to turn it 

 round. 



In all these instances the effect of friction is reduced by dimi- 

 nishing the extent of the sliding motion. 



In balances and other levers the bearing of the lever is in the 

 form of a prism, called a knife-edge, having an angle of about 

 1 20 ; the edge of this prism is accurately ground to a straight 

 line, and rests on a plane horizontal surface of agate. 



The relative motion in this case is one of rolling contact. 



In another class of instruments sliding and rolling are entirely 

 done away with, and sufficient freedom of motion is secured by 

 the pliability of certain solid parts. 



Thus many pendulums are hung, not on knife-edges, but on 

 pieces of watch-spring, and torsion balances are suspended by 

 metallic wires or by silk fibres. The motion of the piece is then 

 affected by the elastic force of the suspension apparatus, but this 

 force is much more regular in its action than friction, and its 

 effects can be accurately taken account of, and a proper correction 

 applied to the observed result. 



12. THE TORSION ROD, OR BALANCE OF TORSION.. 



The balance of torsion has been of the greatest benefit to 

 modern science in the measurement of small forces. The first 



* Communicated to the Royal Society in 1666. See Willis's Principles of 

 Mechanism, 1870, p. 53. 



