TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 275 



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

 divisions, viz. : ft-4.0 



8-37 

 8-38 

 8-38 

 8-36 

 8-36 

 8-58 

 8-33 

 8-31 

 8-47 

 8-33 

 8-33 

 1B.-.38 

 8-44 

 8-38 

 8-40 

 8-37 

 8-40 

 8-25 

 8-38 

 20)16-760 



8' 38 mean value. 



In this instance it will be 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 object 

 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 



6-45 x -05 AOOK 

 *=-83g-=- 0386mm - 



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

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



(ii) 8-38 : 6-45::^| : x inch ; 



6-45 x -00197 -0127 

 *= ~38 =8^- ='001515 inch. 



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

 then suppose the screw-micrometer value for -nnro^h inch =4- 257, 

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



(iii) 4-257 : 6'45 : : '001 : x inch ; 



6-45 x -001 



4-257 



-=001515 inch. 



1 In the number given 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 



