AC C ESSOR Y AP PA R ATUS 



1 1' t IK- an>wei i- required in metrical measurement, then as 1 

 2.V4 nun.. 



4-257 : G-45 :: ('001 x 25-4) : .< mm. ; 

 li-45x-0254 -1638 



(iv) 



lii thi> connection it will be as well to give two example> of 

 scale comparison which are sometimes required. Thus you have a 

 certain interval on a metrical stage micrometer which you know to 

 be accurate, and you wish to compare an English stage micrometer 

 with this -cale in order to find out which particular interval of-j-J^ 

 inch agrees with it. Suppose '05 mm. = 8'38 screw value as above, 

 t hen all that is nece-sary is to find the point to which the screw 

 micrometer must be set in order that it may accurately span the 

 ,,,',,,, inch. Take 1 inch=25'4 mm. as before: then -001 inch = 

 0254. 



(v) -05mm. : -0254mm. :: 8'38 : x screw value; 



0254x8-38 ,__ 

 ./'= =4'257 screw value. 



Oo 



Conversely, if a metrical scale is to be compared with an accurate 

 English one where '001 inch=4'257 screw value, then the screw 

 value for '05 mm. may be found thus: '001 inch='0254 mm. 

 (vi) '0254mm. : '05mm. :: 4'257 : x screw value; 

 05 x 4-257 



0254 



- = 8'38 screw value for '05 mm. 



A cJii'ti/i substitute for the screw m'cnwweier has been devised by 

 Mr. f!. Jackson. It consists in having a transparent arbitrary scale 



A inserted into an or- 



dinary Huyghenian 

 eye-piece in the focus 

 of the eye-lens, so 

 that it will be in the 

 same plane as the 

 magnified image of 

 the object to be 

 measured. It is seen 

 in fig. 215. The 

 method of using it is 

 precisely similar to 

 that of the screw 

 micrometer; the 



value of T(1 \ )(1 inch or 

 ,',, mm., as the case 

 may lie. is found in 

 t erms oft he arbitrary 

 scale. The value of 



I: 



i 



L ,_, 



Fli . '2\:>. Jackson's micrometer. 



''"' "''.i 1 ''-' iii Iti'ins of the same scale is also found, and comparison 

 accordingl. All that need I..- done i.s t,, substitute the terms 





: he ai-hit rai-\ scale for screv \alnes in the preceding examples, and 



they will meet the ca-e. 



