Physical Units and Constants

Authored by: Jane H. Hodgkinson , Frank D. Stacey

Practical Handbook of Earth Science

Print publication date:  September  2017
Online publication date:  September  2017

Print ISBN: 9781138552234
eBook ISBN: 9781315148038
Adobe ISBN:


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There is nothing fundamental about the SI system. It is simply the result of a decision to produce a single universal system of units to replace multiple alternative systems. Modifications/additions have been introduced from time to time and the system is not without its difficulties. In the Earth sciences, the greatest difficulty is experienced in geomagnetism and rock magnetism, but practitioners in these disciplines are constrained by a consensus favouring a common system of units across all science. Some flexibility is assumed, and Table 1.1 presents some SI equivalents to the formal units with multiplying factors in Table 1.2. Thus, the geomagnetic field strength is generally expressed in millitesla, although the formal SI prescription is ampere/metre, both being SI units. Caution is required in the use of the mole as a chemical unit of material quantity because it refers to mass in grams, not the SI unit, kilograms, and is a survivor from the centimetre–gram–second system. Radiation intensity is most conveniently expressed in watts, W/unit solid angle or W/m2 at a specified distance, without reference to its wavelength, avoiding the generally impractical SI definition of the candela. Units applied to radioactive exposure doses are not listed here. Conversions from units in common non-SI systems are listed in Section 1.2 with more details for fossil fuels in Section 24.5.1.

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