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Definition: logarithm from Philip's Encyclopedia

Aid to calculation devised by John Napier in 1614, and developed by the English mathematician Henry Briggs. A number's logarithm is the power to which a base must be raised to equal the number, i.e. if bx = N, then logb N = x, where N is the number, b the base and x the logarithm. Common logarithms have base 10, and so-called natural logarithms have base E (2.71828...). Logarithms to the base 2 are used in computer science and information theory.

Summary Article: logarithm from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

The exponent or index of a number to a specified base – usually 10. For example, the logarithm to the base 10 of 1,000 is 3 because 103 = 1,000; the logarithm of 2 is 0.3010 because 2 = 100.3010. The whole-number part of a logarithm is called the characteristic; the fractional part is called the mantissa.

Before the advent of cheap electronic calculators, multiplication and division could be simplified by being replaced with the addition and subtraction of logarithms.

For any two numbers x and y (where x = ba and y = bc), x × y = ba × bc = ba + c; hence one would add the logarithms of x and y, and look up this answer in antilogarithm tables.

Tables of logarithms and antilogarithms are available that show conversions of numbers into logarithms, and vice versa. For example, to multiply 6,560 by 980, one looks up their logarithms (3.8169 and 2.9912), adds them together (6.8081), then looks up the antilogarithm of this to get the answer (6,428,800). Natural or Napierian logarithms are to the base e, an irrational number equal to approximately 2.7183.

The principle of logarithms is also the basis of the slide rule. With the general availability of the electronic pocket calculator, the need for logarithms has been reduced. The first log tables (to base e) were published by the Scottish mathematician John Napier in 1614. Base-ten logs were introduced by the Englishman Henry Briggs (1561–1631) and Dutch mathematician Adriaen Vlacq (1600–1667).


Mathematics and Computing

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