Another thing we must understand is that the decimal point is not found on a slide rule. For example the 16 on the outer bezel also represents 0.0016, 0.016, 0.16, 1.6, 160, 1600, 16000 etc. A little bit of common sense is needed here, and will become apparent in the explanations below. The slide rule is also not a linear scale but a logarithmic one. This just means the physical distance on the scale between similar intervals is different, for example the distance between 10 and 20 on the outer ring is much further than between 20 and 30. The best thing to do for practice is make different calculations with your watch and check them with a calculator to gain confidence.
First off, let’s start with everybody's favourite calculation: Division.
This calculation is fairly easy actually. Let’s say we want to divide 75 by 12. Maybe it's a shared Christmas gift for someone (a little late I know but a good example). Line up 75 on the sliding outer track with 12 on the fixed inner track. The 10 (in red) on the fixed inner track will point to the answer which is 62.5. Now the decimal place dilemma comes into play.
We know that 62.5 is much too large an answer for our question. So we mentally slide the decimal place across to either 6.25, or further to 0.625. It’s easy to see that 0.625 is way too small to be the answer, so we stick with 6.25 or $6.25 per person. |
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