138 



MICROMETER SCREW. 



[CH. VI 



Jnto contact with a surface of oil or mercury contained 

 in a cup which is raised or lowered by means of a micro- 

 meter screw*. The moment of contact is a very definite 

 one, and with practice it is possible to read to O'Ol mm. 

 If oil is used for the fluid the oil-pot must first be lowered 

 until the needle point .hangs clear, the screw is then 

 cautiously raised until the bright surface of the oil is 

 dimpled at the point of contact. If mercury is used a 

 vertical wire or thread must be hung close to the micro- 



Fig. 25. Exp. 178. 

 (Instrument Company's Catalogue.) 



1 This method was suggested and the apparatus designed b; Mr H. 

 Darwin of the Cambridge Scientific Instrument Company. 



