Suppose three real numbers a, x, and y are related according to the following equation:
x = ay
Then y is defined as the base-a logarithm of x. This is written as follows:
loga x = y
As an example, consider the expression 100 = 102. This is equivalent to saying that the base-10 logarithm of 100 is 2; that is, log10 100 = 2. Note also that 1000 = 103; thus log10 1000 = 3. (With base-10 logarithms, the subscript 10 is often omitted, so we could write log 100 = 2 and log 1000 = 3). When the base-10 logarithm of a quantity increases by 1, the quantity itself increases by a factor of 10. A 10-to-1 change in the size of a quantity, resulting in a logarithmic increase or decrease of 1, is called an order of magnitude. Thus, 1000 is one order of magnitude larger than 100.
Base-10 logarithms, also called common logarithms, are used in electronics and experimental science. In theoretical science and mathematics, another logarithmic base is encountered: the transcendental number e, which is approximately equal to 2.71828. Base-e logarithms, written loge or ln, are also known as natural logarithms. If x = ey, then
loge x = ln x = y