Comparators for Measures of Length. 



2ir 



Fig. 8. 



It was found, after a somewhat extended trial, that this 

 comparator answered the purpose of its construction admir- 

 ably, simply as a comparator, in which the same part of the screw 

 was used to compare different spaces, but it was found inade- 

 quate to the measurement of absolute lengths in terms of the 

 screw. It will be seen from the cut that the screw passes 

 through a short nut {a). As the plate which carries the object 

 to be measured is held against the oval end of the screw by 

 parallel springs, which have a maximum tension of four pounds, 

 the screw has an inevitable tendency to wabble, as the leverage 

 becomes greater by running the screw out its entire length. 



In order to remedy these defects, Mr. Geo. B. Clark, of the 

 firm of Alvan Clark & Sons, constructed for me a comparator 

 of the design shown in the accompanying illustration. The 

 comparator proper consists of a bed-plate, within which is 

 fitted a slide, carried by the precision screw b. The object 



