Slide Rules

I was working “the night shift” last night.  The company I work for is doing some major network upgrades and this kind of stuff is done at night.  I was watching to make sure the the equipment still worked after the upgrades, so it was watch, wait awhile, watch some more and so on as new systems were put online and the old systems were turned off.  I wish I could have done more, but I was watching from half way across the country from where the physical work was being done.

So, to keep myself entertained with something that I could instantly set down, I pulled out an old slide rule.

It sounds like rules involved in using a slide.  But that’s not the case.

It’s this.


This is a Post Versalog slide rule, model 1460.  With it, you can do most any mathematical operation except for addition and subtraction.

  • Multiplication and division? Yes.
  • Exponentiation and logarithms? Yes.
  • Roots? Yes.
  • Trig functions?  Yes.

And, just like a car being made of parts, you can use the above calculations to, with some workarounds, do what is needed.

The slide rule I had in high school was the least sophisticated/expensive one Pickett (another slide rule maker) made.  I could do all the stuff I listed above but I would often have to resort to workarounds to complete my calculations.  Like for most everything, more money equates to more ease of use, and the more expensive slide rules had more features that reduced the need for the workarounds. .

I was about half way through high school when the slide rule to calculator switch happened.  If you think the transition from film to digital cameras was fast, calculators pretty much completely replaced slide rules within two years.

This Post slide rule was given to me by my dad.  Mom was a K&E slide rule fan. I have her slide rule as well.

Going from my high school Picket to either my parents’ side rules is like going from a high school “first car” to a new sports car. And continuing on to the calculator is like going from the new sports car to Star Trek’s Enterprise.

Regardless of the complexity or “fancyness”,  slide rules give you, with some exceptions, three significant figures worth of precision.  This means that 1,152 will be read as 1150, while 6.022140857 is going to be read as 6.02.  The same goes for huge numbers 12,831,451,421 is going to be read as 12,800,000,000.

Normally the loss of precision was not a problem because the result was close enough to be good enough.

0615162004a_resizedFor the cases where close enough was not good enough, handbooks with tables of the results of the calculations were available.  I have one book showing the sine, cosine and tangent functions out to eight significant figures and another book with more tables than just the trig functions, but with results only to six places.

Using one of these books was slow and tedious, but it was better than doing the calculations by hand. Would you rather calculate the value of ln 2.51 or look it up in a table?

It took me about an hour to remember how to use all of the scales on the slide rule.  Am I as efficient as I used to be?  Certainly not.  But I still can do it.  The same goes with the math tables. I’m not fast, but they aren’t either.

Why do I do it?   In my 3rd job, I’m always stressing the ability to degrade gracefully.  Basically this is, “If the fancy system breaks, can you keep going even if it’s not as good as before?  Or will you instantly go from everything to nothing?”

This is just another face to this same thought. If something happens and my calculator doesn’t work, I can keep going.


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