420 



APPLIED SCIENCE 



distances between all parts of a parallel surface. To insure 

 this accuracy mathematical instruments are required. 



481. Micrometer. Accurate mechanical work can be 

 done only when diameters are carefully measured. As a 

 means to this end the micrometer caliper (Fig. 197) is em- 

 ployed to measure the one-thousandth or one five-thousandth 



part of an inch. 

 Its principal 

 parts are the 

 screw, the hub, 



and 

 ble. 



the thim- 



FIG. 197. Micrometer Caliper. This caliper will 

 measure the thickness of articles upwards by 

 thousandths of an inch. The table of decimal 

 equivalents is stamped on the frame. 



The screw of 

 the micrometer 

 is covered by a 

 thimble to pro- 

 tect it from dust 

 and rapid wear. 



By turning the thimble the screw is moved backward or forward, 

 increasing or decreasing the distance between the measuring points, 

 and therefore opening or closing the instrument for larger or smaller 

 diameters. 



One complete revolution of the thimble changes the opening of 

 the caliper .025 in. As the pitch of the screw in the caliper is 40 

 per inch, and the circumference of the thimble is graduated in 

 twenty-fifths, one turn of the screw changes the caliper opening 

 .001 in. 



The heel is graduated in a straight line parallel with the screw 

 length and conforms to the pitch of the latter, each division being 

 .025 in. The fourth division, which is .1 in., is made on the 

 frame with the figure 1 ; the eighth, with the figure 2, and so on. 

 When the thimble is turned one complete revolution, the screw ad- 

 vances -fis in., and -?V of ^V is .001. 



