I U 



CON8I DERATIONS 



variable praMure a* the screw moves forward. A much better arrangement is 

 to make the spring bear, not against the screw itself, but against a nut which 

 fa* to move on the screw, but which is prevented from turning round by a 

 proper bearing. Tliis movable nut always remains at the same distance from 

 the fixed one, so that the pressure of the spring remains constant. This is 

 the arnogement of the micrometer screws in Sir W. Thomson's electrometers. 



11. ON CONTRIVANCES FOR SECURING FREEDOM OF MOTION. 



In many instruments there is a movable part or indicator, the position or 

 motion of which is to be observed in order to deduce therefrom some conclusion 

 with respect to the force which acts upon it. This force may be the weight 

 of a body, or an attractive or repulsive force of any kind ; but, besides the 

 force we are investigating, the resistance called Friction is always acting as a 

 disturbing force. 



If the magnitude and direction of the force of friction were at all times 

 accurately known, this would be of less consequence ; but the amount of friction 

 Is liable to sudden alterations, owing to causes which we can often neither 

 suspect nor detect, so that the only way in which we can make any approach 

 to accuracy is by diminishing as much as possible the effect of friction. The 

 modes by which this is effected are of two kinds. Whenever there is sliding 

 contact, there is friction ; and wherever there is complete freedom of motion 

 tlu-re must be sliding contact ; but by making the extent of the sliding motion 

 small compared with the motion of the indicating part, we may reduce the effect 

 f 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 diminishing 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 120; the edge of this 



* Communicated to the Royal Society in 1666. See Willis's Principles of Mecftaiiism, 1870, p. 53. 



