William OughtredWilliam Oughtred and others developed the slide rule in the 17th century based on the emerging work on logarithms by John Napier. Before the advent of the pocket calculator, it was the most commonly used calculation tool in science and engineering. The use of slide rules continued to grow through the 1950s and 1960s even as digital computing devices were being gradually introduced; but around 1974 the electronic scientific calculator made it largely obsoletelogarithmsJohn Napier Introduction to the Slide Rule

Addition Using a Ruler What addition sum does this show? = = = 7 … Alternatively this could be seen as a subtraction: 5 – 2 = 3 6 – 2 = 4 7 – 2 = 5 …

Addition Using a Ruler What addition sum does this show? = = = 9 … Alternatively this could be seen as a subtraction: 5 – 4 = 1 6 – 4 = 2 7 – 4 = 3 …

Logarithmic Scales We are going to replace the numbers 0, 1,,2, 3 … with Powers of

Logarithmic Scales This leaves us with the logarithmic scale shown below. When we move one unit along instead of adding 1 we are doubling This has the effect of extending the scale from the original 0 to 8 to 1 to 256 If we had picked Powers of 3 it would have been an even larger range …

Logarithmic Scales on a Slide Rule We are now going to put the two logarithmic scale together and see why this is useful

Logarithmic Scales on a Slide Rule Previously we used this to show that = 5 Or 5 – 2 = 3

Logarithmic Scales on a Slide Rule Looking at the logarithmic scale we see that 4 x 8 = 32 Or 32 / 4 = 8

Logarithmic Scales on a Slide Rule If we look at the powers we can see that = 5 It becomes a multiplication because 2 2 x 2 3 = 2 5 We are adding the powers and therefore we are multiplying the numbers

Logarithmic Scales on a Slide Rule If we look at the powers we can see that = 7 It becomes a multiplication because 2 3 x 2 4 = 2 7 Or a division because 2 7 ÷ 2 3 = 2 4

This is a simplified slide rule with just the two scales: Click hereClick here This is complete slide rule (Use the A&B scales which go up to 100 or C&D scales which go up to 10): Click hereClick here If you Flip the middle scale (see top right of the Slide Rule) you can see some convenient conversion scales. D to A is squaring D to K is cubing