How to Use a Slide Rule

							How to Use a Slide Rule
Suppose you want to compute 8+15 but you forgot how toadd. Would it help
if you had two yardsticks? Then you could just line them up so that they
are offset by 8 inches. If the 0 lines up with the 8 then the
corresponding 15 lines up with the 23.
Too bad that only works with addition. Multiplication is harder. The
basis of a slide rule is that this does work with multiplication, we just
need to put a different scale on the yardstick. One so that when 8 lines
up with 1, the 15 lines up with 120. We need the scale to satisfy the
property that f(x)+f(y)=f(xy). Logarithms do that, so we just need to
make a yardstick with a log scale instead of a linear scale. Although
fancier models exist, that's all a basic slide rule is. You can even make
your own.

<Steps
1Convert inches to centimeters step by step.One inch equals 2.54
centimeters. So we need to multiply by 2.54.<
2Align the 1 on the sliding part with 2.54 on the fixed part. Be sure to
use the right scale. If you're using a slide rule likethis, use the
scales labeled C and D (CF and DF will also work).
3Find the number of inches you want on the sliding scale. For example, if
we want 155 inches we look at 1.55 (Slide rules are intended for
multiplying number between 1 and 10. We'll have to keep track of the
decimal point ourselves.)
4Look at the fixed part above 1.55 and read off your answer. I got 3.96.
5Multiplying by the right power of 10, we find that 155 inches equal 396
cm. (Off by about 0.5%)<

Tips
The precision of a slide rule depends on how many distinguishable
calibrations marks you can make on it. The quick and dirty models
described are probably good for about 2 significant figures. Maybe three
if you have good eyes. If you need more precision, remember that better =
longer.

<Things You'll Need
slide rule

Related wikiHows
How to Calculate in Your Head the Month when You Know the Rest of the
Date
How to Convert a Mixed Fraction
How to Add or Subtract Fractions Using Vedic Math
How to Square Numbers Ending in 5