28 



DR A. E. H. TUTTON: 



APPENDIX. 



The Use of Wave-length Rulings as Defining Lines on Standards of Length. 



The delicacy of the method of measurement in wave-lengths, described in the 

 preceding communication, calls for a corresponding refinement in the engraved lines 

 which form the defining limits of the length of a Standard Yard or metre or other 

 line-measure bar. The Imperial Standard Yard is shown in fig. 9. It is a bar of 

 Baily's metal, which consists of 16 parts copper, 2|- parts tin, and 1 part zinc. The 



Fig. 9. The Imperial Standard Yard. 



bar is of 1-inch square section, and is 38 inches long. At positions 1 inch from each 

 end cylindrical holes, |- inch diameter, are bored for a depth of half an inch, the depth 

 of the axis or " neutral plane " of the bar and position of miniimim flexure. In the 

 centre of the base of each of these depressions a gold plug is inserted, the exposed 

 surface of which, lying in the neutral plane, has been truly planed. The defining 

 mark is engraved approximately at the centre of this gold circular surface. Five 

 lines are engraved, two parallel to the length of the bar and three transversely, the 

 appearance of the lines as seen in the microscope being shown in fig. 10. It is the 



Fig. 10. 



Fig. 11. 



Fig. 12. 



Defining lines on Imperial 

 Standard Yard. 



' Tresca " section of platinum- 

 iridium yard. 



Defining line on platinum-iridium 

 yard. 



central one of the three transverse lines (vertical in the figure) which is the defining 

 mark, and the distance between this mark in the depression at one end and the 

 corresponding mark in the depression at the other end is the Imperial Yard. It is 

 identical, within the two-hundredth part of an inch, with the yard of Henry VII. 

 still preserved in the Standards Office. 



In the year 1902 an official copy of the Imperial Standard Yard was constructed in 

 platinum-iridium (90 per cent, platinum with 10 per cent, iridium), of X-shaped 



