STANDARDS OF MEASUREMENT — BRIGGS 165 



Table 1 — Wave length of the red line of cadmium in angstroms ' 



Data 



1895 

 1005-6 



1927 



1933 

 1934-35 



1933 

 1934-35 



Authors 



Michelson and Benoit — 

 Benoit, Fabry, and Perot. 

 Watanabe and Imaizumi.. 



Sears and Barrell 



Sears and Barrell 



Kosters and Lampe 



Kosters and Lampo 



Mean. 



As originally 

 given 



6438. 4722 

 6438. 4690 

 6438. 4685 

 64S8.4711 

 6438. 4709 

 6-438. 4672 

 6-138. 4685 



After (a) cor- 

 rection and 

 (6) adjust- 

 ment to uni- 

 form con- 

 ditions^ 



6438. 4691 

 6438. 4703 

 6438. 48S2 



6438. 4708 



6438. 4709 

 6438. 4672 

 6438. 4685 



6438. 4693 



Difference 

 from mean 



-.0002 

 -f. 0010 

 -.0011 

 -H.0015 

 -I-.OOIG 

 -.0021 

 -.0008 



±.0012 



"Sears. J. E., Sci. Prog., vol. 31, p. 209, 1936. ^ , k 



3 The values originally quoted by the authors are corrected in the fourth column to take account ot sud- 

 sequent conclusions (a) "regarding the values to be attributed to the standards of length employed, and ad- 

 justed (6), so far as the information available permits, to uniform conditions of "normal" air— i. e., dry air 

 at 16° C. and 760-mm. pressure, containing 0.03 percent of COj. 



THE PRIMAEY STANDARD OF LENGTH 



The national primary standard of length is represented by the 

 platinum-iridium meter bar No. 27. Its use is limited to compari- 

 sons with the working standards. A companion bar, No. 21, of 

 identical form and composition, has borne the brunt of extensive 

 comparisons for more than 40 years, particularly in connection with 

 the certification of geodetic tapes for the Coast and Geodetic Survey. 

 The Bureau also owns two other platinum-iridium meter bars of 

 an earlier alloy, one of which is graduated in millimeters. 



The stability of these bars in service reflects the wise judgment of 

 those who were responsible for the selection of the alloy from which 

 the prototypes were made. Meter No. 21, which has been used so 

 much and subjected to the thermal shock of innumerable ice baths, 

 has increased in length about 1 micron during its 50 years of serv- 

 ice. Meter No. 27, the national prototype, has been certified by the 

 International Bureau of Weights and Measures as follows : 



In 1888-89 : 



No. 27=1 meter -1.50 microns at 0° C. 

 In 1921-23 : 



No. 27=1 meter- 1.48 microns at 0° C. 



These equations indicate that within the limits of measurement 

 the length of meter bar No. 27 has remained invariable in relation 

 to the international prototype for the period covered. This fact 

 alone does not of course preclude the possibility that both bars are 

 drifting. The conclusion that they are not is supported by other 

 intercomparisons and can be examined in another way. During a 

 period of 40 years, various determinations have been made of the 

 length of the meter in terms of wave lengths of the red radiation 



