ANALYST'S COLUMN Standards—Physical Constants. At present measurements m a y be made in terms of national standards of mass, length, and time with probable errors of about 3 parts in 10° for the 1-kg. mass. 3 parts in 10s for the 1-meter length and 1 part in 10s for the 1-second time. As length and time can be expressed with even greater precision in terms of physi cal wave lengths and atomic transition frequencies, it is desirable to relate these standards to such physical constants. Although fundamental standards are quite precise, precision of derived stand ards is considerably less. T o improve them requires an exact knowledge of physical constants. For example, if cur rent and potential difference standards are to be improved, a more exact knowl edge of g, the acceleration due to gravity, is needed, since this constant enters into the measurement through the force ex erted by the current. A new method of determining g, based on timing a body in free fall, will have an expected accu racy of 2 ρ .p.m., which is better than the physical pendulum method. Work on a special group of universal physical constants, including the velocity of light, electronic charge and mass, fine structure constant, Faraday, Rydberg constant, Avogadro's number, Planck's constant, and Bollzmann's constant, shows that they all have a close inter relationship. However, the lack of knowl edge of the values of these constants and lack of guidance in selecting the best fundamental quantities, makes the selec tion of three or four universal constants premature. At present it is more practical to rely on such units as the optical wave length standard, determined to parts in 107 and precise to parts in 10s and the cesium frequency standard, recently determined to 2 parts in 10° and precise to a fewparts in 1010. An improved optical wave length stand ard increases accuracy and precision by increasing the sharpness of t h e wave length line. This is accomplished by reducing the Doppler line width. Research was also reported on the determination of the proton gyromagnetic ratio in a weak field by a freeprecession technique. This is used as a standard in calibrating magnetic fields. The new work has effected a fourfold im provement in the 22 p.p.m. accuracy heretofore attainable. This work will help improve accuracy of universal phys ical constants and in checking the con stancy of the ampere. T h e 36 technical papers presented at the conference will appear in a special issue of Transactions of the IRE Profes sional Group on Instrumentation. Copies are available at S3.00 (nonmembers) from the Institute of Radio Engineers, Inc., 1 East 79th St., New York 31, Ν . Υ.
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