The Slide Rule – Part 1

A logarithmic slide rule


“Time is a wayward traveller, who sometimes rides posthaste through thick and thin…”

From On The Making Of Gardens by Sir George Sitwell

For some strange reason that opening sentence, by Sir George, has been rolling around my mind for nearly twelve months. Perhaps it’s time to try and explain why.

I grew up in northern Stoke-on-Trent on the border of southern Cheshire. I could literally cross the street and be in a different county. During the summer I would ride my second hand BMX bike up to the local reservoir. On a clear day you could look across the Cheshire plains and see the Jodrell Bank radio telescope. Turning to the north east you’d see Mow Cop Castle.

On the flip side of the coin was the abject poverty. When I was very young a travelling funfair would show up at the local cricket ground around the end of October. I remember the toffee apples and the magic of the candy floss machine.

On the way back home we’d pass a run down corner terraced house with a rusty old fence. It was dark and a little child, my age, ran up to the fence and uttered five words:

“Will you be my friend?”

And before I could formulate a response my dad whisked me away. The poor guy was not born lucky. I can still picture him sitting alone in the corner of the classroom trying to count little coloured cubes. I used to watch him and want to go and help.

So let us start at the beginning and learn to count like our poor friend.

Addition And Subtraction

From cars, trains, ships, aeroplanes to space travel and modern digital computers the humble slide rule accelerated technological development in the twentieth century.

Let’s make a simple slide rule to perform addition and subtraction:

A simple linear slide rule

In the photo above I’ve drawn a linear scale between 0 and 7. By sliding the B rule’s 0 underneath the A rule’s 2 we see that we can add or subtract by 2.

Addition and subtraction by 2

If we look at the B scale, say the number 3, and then look at the A scale we get 5. We’ve added 2+3. Inversely, look at the number 6 on the A scale and we get 4 on the B scale. Simple. Well, it does take a bit of getting used to!

On a quick side note you’ll notice that the slide rule gives you multiple answers to the same question. It’s really a parallel computer.

A linear slide rule is all very well, but if we want to reach the moon we’re going to need something more than just addition and subtraction, which we can all do.

In part 2 we’ll look at the calculation to derive a logarithmic scale which opens up the world of multiplication, division, roots and powers.

Think about our poor little friend trying to learn to count from one to ten. For the moment remember this: ten is your friend.

© text & images Doc Mike Finnley 2023