What is the Oughtreds slide rule?

Answer 1

Oughtred's slide rule is two mated Gunter scales.

A man named Gunter devised the Gunter scale. It is like a ruler, except that the divisions represent the values associated with logarithms of the distances from the end. So on a ten inch long scale, the numbers would be at the following distances from the left end:

1. at 0 inches
2. at 3.01 inches
3. at 4.77 inches
4. at 6.02 inches
5. at 6.99 inches
6. at 7.78 inches
7. at 8.45 inches
8. at 9.03 inches
9. at 9.54 inches
10. at 10 inches

By using dividers, this ruler-like scale could be used for multiplication.

Oughtred realized that if a person had two Gunter scales, one with the graduation lines and numbering along the top edge, and the other with them along the bottom edge, they could be used to do math without need for dividers. They were held together by mitering one into the other, and so the slide rule was born.

We can multiply two numbers by adding their logarithms and finding what the result is the logarithm of. For example, we could put the 1 on the top scale at the point where 2 is on the bottom scale. If we then look where the 3 is on the top scale, we have added the distances marked 2 (3.01 inches from the end) and 3 (4.77 inches form the end), and found 6 (at a point 7.78 from the end). So 2 x 3 = 6.

The early slide rules were usually two feet long, and were used in British customs houses for calculating taxes on goods whose weights needed conversion. The original form of the slide rules was used as a part of tools until into the twentieth century. The simple slide rules were improved a number of times, most notably by a French army officer named Mannheim, who added a cursor during the 1870s, and his name is often used to describe the simplest type used in the twentieth century by engineers.