CH. VI] 



MICROMETER SCREW. 



151 



one, and it is not difficult to read to O'Ol mm. If oil is the 

 fluid used, the vessel 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 



FIG. 27. Exp. 178. 

 From the Cambridge Instrument Company's Catalogue. 



thread must be hung close to the micrometer, between it 

 and the light, and the moment of contact of the needle- 

 point and the mercury determined by the distortion of the 

 image of the thread seen by reflection in the mercury. In 

 either case the apparatus must stand on a steady table. 



