Page 575 - Engineering Electromagnetics, 8th Edition

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APPENDIX

Units

We describe first the International System (abbreviated SI, for Syst`eme International
d’Unit´es), which is used in this book and is now standard in electrical engineering
and much of physics. It has also been officially adopted as the international system
of units by many countries, including the United States. 1
The fundamental unit of length is the meter, which was defined in the lat-
ter part of the nineteenth century as the distance between two marks on a certain
platinum-iridium bar. The definition was improved in 1960 by relating the meter to
the wavelength of the radiation emitted by the rare gas isotope krypton 86 under
certain specified conditions. This so-called krypton meter was accurate to four parts
per billion, a value leading to negligible uncertainties in constructing skyscrapers or
building highways, but capable of causing an error greater than one meter in deter-
mining the distance to the moon. The meter was redefined in 1983 in terms of the
velocity of light. At that time the velocity of light was specified to be an auxiliary
constant with an exact value of 299,792,458 meters per second. As a result, the latest
definition of the meter is the distance light travels in a vacuum in 1/299,792,458
of a second. If greater accuracy is achieved in measuring c, that value will remain
299,792,458 m/s, but the length of the meter will change.
It is evident that our definition of the meter is expressed in terms of the second,
the fundamental unit of time. The second is defined as 9,192,631,770 periods of
the transition frequency between the hyperfine levels F = 4, m F = 0, and F = 3,
2
m F = 0of the ground state s 1/2 of the atom of cesium 133, unperturbed by external

1 The International System of Units was adopted by the Eleventh General Conference on Weights and
Measures in Paris in 1960, and it was officially adopted for scientific usage by the National Bureau of
Standards in 1964. It is a metric system, which interestingly enough is the only system which has ever
received specific sanction from Congress. This occurred first in 1966 and then again in 1975 with the
Metric Conversion Act, which provides for “voluntary conversion” to the metric system. No specific
time was specified, however, and we can assume that it will still be a few years before the bathroom
scale reads mass in kilograms and Miss America is a 90–60–90.

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