In mathematics, the common logarithm is the logarithm with base 10. It is also known as the decadic logarithm, named after its base. It is indicated by log₁₀(x), or sometimes Log(x) with a capital L (however, this notation is ambiguous since it can also mean the complex natural logarithmic multi-valued function). On calculators it is usually "log", but mathematicians usually mean natural logarithm rather than common logarithm when they write "log". water flow File links The following pages link to this file: Common logarithm ... water flow File links The following pages link to this file: Common logarithm ... Euclid, Greek mathematician, 3rd century BC, as imagined by by Raphael in this detail from The School of Athens. ... Above is the graph plots of Logarithms to various bases: is to base e, is to base 10, and is to base 1. ... This diagram does not represent a true function; because the element 3, in X, is associated with two elements b and c, in Y. In mathematics, a multivalued function is a total relation; i. ... Leonhard Euler, one of the greatest mathematicians of all time A mathematician is a person whose primary area of study and research is the field of mathematics. ... The natural logarithm, formerly known as the hyperbolic logarithm, is the logarithm to the base e, where e is equal to 2. ...

Before the early 1970s, hand-held electronic calculators were not yet in widespread use. Because of their utility in saving work in laborious calculations by hand on paper, tables of base-10 logarithms were found in appendices of many books. Such a table of "common logarithms" giving the logarithm of each number in the left-hand column, which ran from 1 to 10 by small increments, perhaps 0.01 or 0.001. There was no need to include numbers not between 1 and 10, since if one wanted the logarithm of, for example, 120, one would know that The 1970s decade refers to the years from 1970 to 1979. ... A calculator is a device for performing calculations. ... Before calculators were cheap and plentiful, people would use mathematical tables —lists of numbers showing the results of calculation with varying variables— to simplify and drastically speed up computation. ... Above is the graph plots of Logarithms to various bases: is to base e, is to base 10, and is to base 1. ...

The very last number ( 0.079181)—the fractional part of the logarithm of 120, known as the mantissa of the common logarithm of 120—was found in the table. (This stems from an older, non-numerical, meaning of the word mantissa: a minor location of the decimal point in 120 tells us that the integer part of the common logarithm of 120, called the characteristic of the common logarithm of 120, is 2.

Similarly, for numbers less than 1 we have

The bar over the characteristic indicates that it is negative whilst the mantissa remains positive. Negative logarithm values were rarely converted to a normal negative number (−0.920819 in the example).

Numbers are placed on slide rule scales at distances proportional to the differences between their common logarithms. By mechanically adding the distance from 1 to 2 on the lower scale to the distance from 1 to 3 on the upper scale, one can quickly determine that 2 x 3 = 6.

In addition, slide rules work by using a logarithmic scale. Image File history File links Slide_rule_example2. ... Image File history File links Slide_rule_example2. ... A typical 10 inch student slide rule (Pickett N902-T simplex trig). ... A logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself. ...

History

Common logarithms are sometimes also called Briggsian logarithms after Henry Briggs, a 17th-century British mathematician. Henry Briggs (February 1556 - January 26, 1630) was an English mathematician notable for changing Napiers logarithms into common/Briggesian logarithms He was born at Warley Wood, near Halifax, in Yorkshire Enland. ... (16th century - 17th century - 18th century - more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601-1700. ...

Because base-10 logarithms were most useful for computations, engineers generally wrote "log(x)" when they meant log₁₀(x). Mathematicians, on the other hand, wrote "log(x)" when they mean log_e(x) for the natural logarithm. Today, both notations are found. Since hand-held electronic calculators are designed by engineers rather than mathematicians, it became customary that they follow engineers' notation. So ironically, that notation, according to which one writes "ln(x)" when the natural logarithm is intended, may have been further popularized by the very invention that made the use of "common logarithms" far less common, electronic calculators.

Numeric value

The numerical value for logarithm to the base 10 can be calculated with the following identity.

as procedures exists for determining the numerical value for logarithm base e and logarithm base 2.

• Natural logarithm#Numerical value
• Binary logarithm#Numerical value

Alternatively below is a correct but inefficient algorithm that can calculate the common logarithm to an arbitrary number of decimal places. ^[1] The natural logarithm, formerly known as the hyperbolic logarithm, is the logarithm to the base e, where e is equal to 2. ... Plot of log2 x In mathematics, the binary logarithm (log2 n) is the logarithm for base 2. ...

 #!/usr/bin/python from __future__ import division import math # Calculates the logarithm (of any base > 1) of a positive number # to the an arbitary number of decimal places. The accuracy is # subjected only to limitation of the floating point representation. def log(X,base=math.e,decimalplace=12): integer_value=0 while X < 1: integer_value = integer_value - 1 X = X * base while X >= base: integer_value = integer_value + 1 X = X / base decimal_fraction = 0.0 partial = 1.0 # Replace X with X to the 10th power X = X ** 10 while decimalplace > 0: partial = partial / 10 digit=0 while X >= base: digit = digit + 1 X = X / base decimal_fraction = decimal_fraction + digit * partial # Replace X with X to the 10th power X = X ** 10 decimalplace = decimalplace - 1 return integer_value + decimal_fraction if __name__ == '__main__': value = 4.5 print " X =",value print " 6 decimal places LOG(X) =",log(value,base=10,decimalplace=6) print " 9 decimal places LOG(X) =",log(value,base=10,decimalplace=9) print "12 decimal places LOG(X) =",log(value,base=10,decimalplace=12) # Sample Run # # $ python log.py # X = 4.5 # 6 decimal places LOG(X) = 0.653212 # 9 decimal places LOG(X) = 0.653212513 # 12 decimal places LOG(X) = 0.653212513775 

An approximation for simpler calculators

Early electronic calculators did not have the ability to calculate logarithms, but many could extract square roots. There is a curious approximation to the common logarithm that can be made on such a calculator. If a number has its square root taken 11 times, is reduced by 1, and is then multiplied by 889, then this is an approximation of the common logarithm of that number, of which accuracy varies. For a wide range of numbers from 10⁻¹⁷ to 10⁺¹⁸, this is accurate to within 1%. In other words:

It is based on the fact that 889 ln 10 ≈ 2048 and ln x ≈ x − 1 for x ≈ 1.

References

1. ^ [1]

See also

• Jurij Vega
• slide rule
• History of logarithms