Calculating Machines  Abacus  Napier's bones  Slide Rule  Logarithms  Calculator
A slide rule is used for calculation. Before electronic calculators became cheap enough for school children to use, we had to use slide rules, and very irritating they were too! They had various functions, and even the simplest slide rule had more than one scale, but here is a simplified slide rule with only one scale for you to try. The bottom scale moves and the top scale stays still. You can select a multiple, and then adjust it further if you wish, using the arrows (big arrows move it further than small ones). If you select x2, then the bottom part slides along so the 1 is against the top 2. If you look along the bottom, the 2 is against the top 4, the 3 is against the top 6, and so on. So you can use it to multiply by 2. This isn't very impressive, but you can click on the arrows to slide along until the bottom 1 is against the top 2.1, and now you are multiplying by 2.1 by just reading off the values on the top slide. The irritating thing is that most of the marks don't line up, so you can't read off the value. You have to estimate how the final answer is.

When using this slide rule, you may find that it jumps to the other end of the scale. This is because you tried to move it outside the slide rule. The scale from 10100 is exactly the same as from 110, apart from the number being ten times more. You use the slide rule to work out the figures, then work out where the decimal point should go afterwards. This means that you can use the slide rules for calculations less than 1 or greater than 100.
This slide rule is not completely accurate. A slide rule is an analogue device, so each mark can be in exactly the right place. Computers are digital, and in particular, a computer screen has pixels, and you can't show anything less than a pixel. Some of these marks should be half a pixel (or less) to the left or right  sorry about that! In fact, all slide rules are only accurate to 2 or 3 significant figures.
Once you have tried to multiply some numbers on this, you may understand why electronic calculators replaced slide rules! Below is a photo of a real slide rule in use, with a closeup detail. The top two scales go from 1 to 100 and the bottom from 1 to 10. It is set up to calculate x4 on the top and x2 on the bottom, and the closeup lets you read off some values.
This website has interactive sliderules based on photos of real slide rules.
© Jo Edkins 2007  Return to Numbers index