In 1942, William F. Meggers of NBS developed an electrodeless lamp using the isotope mercury-198. Subsequent models were made with this isotope distilled from proof gold exposed to neutrons in a chain-reacting pile at Oak Ridge. The 13 Meggers lamps initially distributed in 1951 were capable of calibration to a precision of 1 part in 100 million, as opposed to the 1 part in 10 million possible with the standard meter. In 1960, the International Committee on Weights and Measures redefined the international standard of length as 1 650 763.73 wavelengths of the orange-red radiation of krypton-86. However, because of the simplicity of operation of the mercury-198 lamp for laboratory length measurements, it was used more frequently than the krypton-86 and the cadmium lamps.
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