568 



TABLES 610-625A.— STANDARD WAVELENGTHS 17r - 102 AND 

 SERIES RELATIONS IN ATOMIC SPECTRA * 



Primary standard of wavelength. — The red radiation, 6438.4696 A, 

 emitted by a cadmium lamp of Michelson type was first chosen in 1907 by the 

 International Union for Cooperation in Solar Research 177 as a primary stand- 

 ard of wavelength and definition of the angstrom as a unit of wavelength meas- 

 urement. This primary standard was adopted in 1922 by the International As- 

 tronomical Union 178 and in 1927 by the International Committee on Weights 

 and Measures 179 with the statement that the wavelength of this radiation is 

 6438.4696 x 10 -10 meters when the light is propagated in dry air at 15°C 

 (hydrogen thermometer) at a pressure of 760 mmHg, gravity being 980.665 

 cm/sec 2 . 



Specifications for the standard cadmium lamp were last revised in 1935 ; 18 ° 

 they designate that the lamp must be Michelson H-type with internal elec- 

 trodes, excited with continuous or alternating current of industrial frequency, 

 maintained at a temperature near 300° C (never exceeding 320° C) and contain 

 air under a pressure between 0.7 and 1.0 mmHg at that temperature. The con- 

 striction must not be less than 2 mm diameter and the current must not exceed 

 7 milliamps/mm 2 . 



A summary of nine directly measured values of the wavelength of the red 

 radiation of cadmium in terms of the meter has been given by H. Barrell 181 

 as in Table 612. 



177-W2 For f ootnotes 177_192, see p. 578. 



* Data furnished and arranged by W. F. Meggers, National Bureau of Standards. 



TABLE 610.— PRELIMINARY VALUES OF Hg 168 WAVELENGTHS 

 IN ANGSTROMS 



International secondary standards of wavelength from neon, krypton, and 

 iron spectra. — Spectroscopic secondary standards of wavelength are derived from the 

 primary standard (Cd 6438.4696 A) by means of the Fabry- Perot interferometer. The 

 existing international secondary standards represent the mean of three or more independent, 

 concordant values adopted by the International Astronomical Union. All values of sec- 

 ondary standards of wavelength are valid for normal air (15°C and 760 mmHg). The 

 most precisely determined secondary standards of wavelength have been obtained from 

 discharge tubes of the Geissler type containing neon or krypton gas at a pressure not ex- 

 ceeding 15 mmHg. In 1935 the International Astronomical Union 183 adopted 8-figure 

 values of 20 neon wavelengths with the reservation that they apply only to the conditions 

 under which they were determined, viz, with interferometers of high resolving power but 

 plate separations not exceeding 40 mm. 



Ma For reference, see p. 578. 



TABLE 611.— NEON SECONDARY STANDARD WAVELENGTHS 

 IN ANGSTROMS 



5852.4878 6074.3377 6266.4950 6532.8824 



5881.8950 6096.1630 6304.7892 6598.9529 



5944.8342 6143.0623 6334.4279 6678.2764 



5975.5340 6163.5939 6382.9914 6717.0428 



6029.9971 6217.2813 6506.5279 7032.4127 



New values of 20 krypton lines as secondary standards of wavelength were adopted in 

 1935 by the International Astronomical Union. 184 See Table 614. 



184 For reference, see p. 578. 

 SMITHSONIAN PHYSICAL TABLES 



