TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 275 



01 . The following are the actual values obtained for each of the '05 



divisions, viz. : 



o'4U 



8-37 

 8-38 

 8-38 

 8-36 

 8-36 

 8-58 

 8-33 

 8-31 

 8-47 

 8-33 

 8-33 

 8-38 

 8-44 

 8-38 

 8-40 

 8-37 

 8-40 

 8-2-", 

 8-38 



8 '38 mean value. 



In this instance it will lie seen that the last division. 8'38, agrees 

 with the mean, and is the best for all future use. 1 



Having thus obtained a screw-micrometer value for a certain 

 known interval, the screw-micrometer value for any other objivt 

 being known, the size of the object may be found by simple propor- 

 tion ; thus. viz. if 8'38 is the screw-micrometer value for '05 mm. 

 and 6 - 45 that for a certain object, the size of the object is 



(i) 8-38 : 6-45 :: -05 : a; mm. ; 



6-45 x -05 n9QK 

 - = - ='0o85 mm. 



8'38 



If the answer is required in fractions of an English inch, all that 

 we need remember is that 1 inch=25 - 4 mm. ; then 



.|i-, 

 (ii) 8-38 : 6'45:: o _ : ..: inch ; 



6-45 x -00197 -0127 

 .<= -= =-001515 inch. 



8-38 s-38 



If the stage-micrometer is ruled in fractions of English inches, 

 then suppose the screw-micrometer value for , nVnth inch = 4-2.">7. 

 and that for the object=6'45 as before. 



(iii) 4-257 : 6-45:: -001 : scinch; 



6-45 x -001 . 



._=- ='001515 inch. 



4-257 



1 In the number yiven for screw value the whole number stands for a complete 

 revolution or number of revolutions of the screw head, and the decimal, the portion 

 of a revolution read off beyond this. 



T2 



