42 



SECTION 3. MEASUREMENT. 



WEST GALLERY, GROUND FLOOR, ROOMS H. K. 



ISPECIAL COLLECTIONS. 



COLLECTION OF STANDARD MEASURING APPARATUS CONTRIBUTED 

 BY THE STANDARDS DEPARTMENT OF THE BOARD OF TRADE. 



A. Comparing Apparatus, Sf-c.for Standard Weights and 

 Measures. 



169. Comparing Apparatus for End-Standards of length. 

 Used by Mr. Sheepshanks in the work of the Commission for 

 Restoration of the Imperial Standards, 1844-1850. Constructed by 

 Troughton and Simms. 



The standard, and compared end-bars are placed successively on the V 

 supports, with one defining end in contact with the left hand stud and the 

 other defining end with the suspended gravity-piece interposed between it 

 and the screw on the right hand. The micrometer screw is to be gently 

 pressed forward until it just holds up the gravity piece in position, thus 

 ensuring constant pressure for each observation. The readings of the micro- 

 meter being taken, the difference of the two readings shows the difference in 

 length of the two end bars to less than O'OOOl inch, which is the value of 

 one division of the micrometer. 



To obtain results with scientific precision the temperature of the measuring 

 axis of each bar during the comparison should be known, as well as its rate of 

 expansion. The temperature and length of the bar connecting the stud and 

 gravity piece and of the metal of the apparatus should also be constant. 



170. Two Lever Frames, with rollers for supporting stand- 

 ard bars. Such lever frames are used for supporting all the Im- 

 perial Standard yards made by the Commission for restoring the 

 Standards. Constructed by Troughton and Simms. 



Each bar is supported on the eight rollers of the two lever-frames, which 

 are placed symmetrically under the bar, so that the upward pressure of each 

 of the eight different rollers is necessarily equal, and the length between the 

 defining points of the bar is not altered by its flexure. Equal intervals of 



length of bar 

 supports = . > where n is the number of supports. 



171. Double Micrometer Microscope for comparing the 

 smaller subdivisions of standards of length. Constructed by 

 Troughton and Simms. 



It has a movable eye-piece with a double lens, sliding upon a horizontal 

 plate, and two micrometers ; and has two object-glasses, each with a double 



