GENERAL CONSIDERATIONS. 



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io. SCREW MOTION. 



The adjustments of instruments are to a great extent made by 

 means of screws. In the case of levelling screws, which bear the 

 weight of an instrument, the thread of the screw is always in con- 

 tact with its proper bearing in the nut ; but in micrometer screws 

 it is necessary to secure this contact by means of a spring. This 

 spring is sometimes made to bear against the end of the scr6w, or 

 a shoulder turned upon it ; but this arrangement causes a variable 

 pressure as the screw moves forward. A much better arrangement 

 is to make the spring bear, not against the screw itself, but against 

 a nut which is free to move on the screw, but which is prevented 

 from turning round by a proper bearing. This movable nut 

 always remains at the same distance from the fixed one, so that 

 the pressure of the spring remains constant. This is the arrange- 

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



ii. 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 investi- 

 gating, the resistance called Friction is always acting as a dis- 

 turbing 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 efiect 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 there must be sliding contact; but by making 



