Slide rule

This rather specialist post will interest those over 70 or with an interest in the history of technology. As it has happened in my lifetime and I’ve been much involved I’ve chosen to write about it. A slide rule is a rather simple device consisting of a ruler marked in logarithmic numbers with a sliding central portion also marked logarithmically and with a sliding transparent bar on the outside of the ruler ( the cursor with a marked line ). This device enables multiplication and division to be done approximately but quickly. It also, depending on the complexity of the rule, enables other calculations such as reciprocals, squares, cubes and others.

It works simply as a way of adding or subtracting logs or otherwise manipulating them.

When I was a young man the slide rule was the mark of the professional scientist or engineer. It enabled quick approximate calculations which could be followed up if necessary by more precise calculations.

I was started early by the gift of a six inch slide rule by a distant relative. I was very proud of it. My second ( maybe third ) cousin Jarve was an engineer with Dorman diesels at Stoke. His actual first name was Jarvis; a name I’ve never seen elsewhere. We lived near several relatives and he would arrive for a few days visiting all in turn. Toys were in short supply in the post war years and he had made things like a model farmyard for me when I was younger. I was about 11 when he gave the slide rule and fortunately could follow the instructions to get what seemed rather magical results. It was not really acceptable at school, certainly not until sixth form, which required laborious but precise calculation.

Most professional slide rules were 12 inches and I felt very much a second class citizen in the sixth form until on visiting Birmingham with Annette on our first day out together I took the plunge and bought the cheapest 12 inch rule available. Really all gave similarly approximate results but more expensive rules were had more scales and were made of better materials.

I used this new rule all through my undergraduate days. Even during my industrial experience with Bostik, calculations were done on rotary calculators which were simply large  slide rules wound into a cylinder; ours was equivalent to about 36 inches I recall. The accuracy was felt to be sufficient for our purposes. I remember being slightly surprised. The slang name for a slide rule was guessing stick.

When I started into a fairly prosperous lab in the late 60’s electronic calculators were just starting to replace large mechanical or electromechanical calculators. At first these were the size of large typewriters and rather expensive. I recall agonising whether our budget could stretch to a calculator which would compute square roots. With hindsight whatever we decided our purchase was premature

Within a very few years electronic calculators fell rapidly in price and size. As a research student at a poor institution I was reduced to old fashioned mechanical calculators for a while. At the rich company there were even calculators with programmable memory and even memory storage. These latter were expensive and demanded programming in machine code. I wasted many hours programming. I had little success with high level languages like FORTRAN but this seemed like fun.

 Meanwhile slide rules became things of the past. By the time my children were into their teens I found somewhere selling off slide rules at ridiculously cheap prices.  I bought several  but my children were not interested. A fairly cheap electronic calculator would produce accurate and precise calculations for a few button presses and they were quite uninterested in anything else.

Tidying up I came across my cache of bargain slide rules. I tried Alice ( 11 ) and Ben ( 8 ) and they showed some interest. I made the mistake of showing them the most complex and they  found the range of scales confusing. They have taken the ruler away but whether they will ever use it I doubt. They are true members of the  digital generation and a bit of engraved plastic pales before their tablets and games consoles.

Slide rule picture