The strength of an earthquake is measured by taking the common logarithm of the energy emitted at the quake.
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The common logarithm is the logarithm with base 10.
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Other units include the nat, which is based on the natural logarithm, and the hartley, which is based on the common logarithm.
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It is based on the common logarithm of ratios-10 times the common logarithm of a power ratio or 20 times the common logarithm of a voltage ratio.
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More precisely, the order of magnitude of a number can be defined in terms of the common logarithm, usually as the integer part of the logarithm, obtained by truncation.
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The most common unit of information is the bit, based on the binary logarithm.