Using A Logarithm Table
| Field | mathematics, computation |
| Went Obsolete | 1960-1980 |
| Made Obsolete By | The calculator, computers, global positioning systems |
| Knowledge Assumed | A tiny bit of theory to know what you're doing, the ability to multiply or add two numbers, and the ability to read off numbers from a table. |
| When useful | performing calculations involving powers, or the multiplication of large numbers or numbers with significant degrees of precision. Was often used for navigation based on the stars, and more generally for astronomy. |
Logarithm tables were quite useful for computing powers of numbers. Recall that log x^y = y log x, and that e^(ln x)=x. Thus if you wanted to compute x^y you could first look up log x, multiply by y to get a quantity say 'w', and then look up e^w or 10^w depending on which base you were using. More generally, log(x * y) = log(x) + log(y) and so to multiply two large numbers, you can use a similar procedure.
Conversely, if x was a number and y was a variable in an equation number^variable=number, a similar procedure would allow you to find the value of the variable.
It could also be used to convert to and from quantities expressed on a logaritmic scale e.g. pH or the richter scale; or to compute quantities defined by functions which depended on the logarithm of a parameter 'x'. It saved time compared to traditional long multiplication for large numbers.
