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New use for a slide rule

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john halfpenny22/05/2021 19:19:37
197 forum posts
27 photos

I don't know why I still have it; calculators were permitted at college from 1975. However when needing a pair of gears to achieve a certain ratio, I realised that by setting that ratio with the slide I could read along and very quickly see if any whole numbers coincided - whole numbers being the equivalent of the number of teeth on each gear. A quick visual method of selecting available gears from my set, and I was able to settle on 32/49 with 0.02% error, which is fine for my requirement. There are also online calculators, but they were not suitable for my purpose this time.

not done it yet22/05/2021 19:33:20
6499 forum posts
20 photos

Nothing wrong with a slide rule. I still use mine very occasionally, even if only as a demonstration to the younger generation. They have been used for designing aeroplanes (amongst many other things) in the past, so good enough accuracy for most of my uses.

We mostly used the cylindrical versions, at work back in the 1960s.

Brian Wood23/05/2021 10:48:26
2495 forum posts
39 photos

I remember the cylindrical slide rules as well and used them in the process control lab I was working in at the time. They gave better accuracy by virtue of the longer scale length that was available compared to the linear models


Douglas Johnston23/05/2021 11:00:48
763 forum posts
34 photos

When I did my engineering degree in the sixties a slide rule was one of the first things I had to buy. I too still have it although it has not been out of the box for a very long time. It was amazing what one could do with it but I would hate to go back to using it.


Andrew Tinsley23/05/2021 11:07:37
1513 forum posts

I have several slide rules and still use them. I find it quicker than using a calculator.


Nick Clarke 323/05/2021 11:08:34
1314 forum posts
53 photos

Like many others I have not really used my slide rule, bought while at school, since calculators came in although when I started to read Mechanical Engineering in the early seventies there were four HP calculators in the department for student use - all securely bolted down to a desk.

My A level chemistry analysis practical was to identify vanillin (C8H8O3) and the fine white powder got everywhere - and my plastic slide rule still has a faint smell of it to this day!

Speedy Builder523/05/2021 11:16:58
2484 forum posts
196 photos

It gets really interesting when you use the Log and Trig functions. Dad - 40 years designing aircraft from wooden woders through to TSR2 an Concord(e) never had a digital calculator, and said it was easy to work out sections of stressed parts, rivets etc as you can see the next gauge/ size just to the side of your calculated value.

Nigel Graham 223/05/2021 11:51:55
1894 forum posts
26 photos

Yes- still have my Thornton slide rule, plus the one inherited from Dad! Calculators were only just appearing in regular shops in my late-1960s A-Levels days, so slide-rules and books of tables were the norm.

I recall quite a few people saying they found that instant-constant nature of the slide-rule a distinct advantage for repeated calculations.

One major slide-rule maker was Blundell-Harling, based not far from me in Weymouth. They produced two types especially to sel to schools. One was a pocket-sized 6 inches long that schools could issue to pupils. The other was for the teachers, more like six feet long - to demonstrate to the class! Unlike so many firms that failed to spot what was happening in their own field, when calculators started to make slide-rules "obsolete" BH switched to making drawing-boards, rules, scales, etc., and happily are still a local, independant firm very much in business, as a quick look at their web-site shows!


One night some years ago now I was unable to find my calculator anywhere, for a rather awkward engineering "sum" . Slide-rule? Could not find that either. Had to resort to logarithms. Bought new calculator the next day - and the orignal magically re-appeared three weeks later.

My work in the last 3 or 3 decades brought me into intimate contact with the logs I'd been taught merely as times-sums tools at school, and finally brought me to understand them! I think.

Alan Johnson 723/05/2021 13:00:54
106 forum posts
16 photos

I have my Grandfather's wooden Faber slide rule. The slide is missing. He was born in 1875 and worked in Glascow as an mechanical engineer in a firm that manufactured roof trusses and the like before moving to Australia in 1920. They had planned to come in 1914, but were delayed a bit by some event or another in Europe!

A couple of years ago I was showing the slide rule to my son-in-law who is a mathematician and statistician. His response was "what is it?" After I explained how it worked, and that "the battery had never been changed" he grasped the simplicity of the device.

I decided not to mention ".... log tables" as I felt that would be a bit much for him!

How quickly is technology forgotten!

duncan webster23/05/2021 13:29:13
3696 forum posts
69 photos

I had 3 slide rules until recently, gave 2 to a chap who has a huge collection. Honestly, some people will collect anything. Used to be the engineer's trade mark, but very easy to be out by factor(s) of 10. I wonder how many disasters that has caused in the past

Michael Gilligan23/05/2021 13:36:48
19561 forum posts
993 photos
Posted by Alan Johnson 7 on 23/05/2021 13:00:54:


A couple of years ago I was showing the slide rule to my son-in-law who is a mathematician and statistician. His response was "what is it?" After I explained how it worked, and that "the battery had never been changed" he grasped the simplicity of the device.

I decided not to mention ".... log tables" as I felt that would be a bit much for him!

How quickly is technology forgotten!


When our daughter [now a lecturer] wanted one of those new-fangled electronic calculators for use at school ... I insisted that she first explained to me how multiplication on a slide-rule works.
[ this was long before calculators were common in schools ]

Sometimes you have to be cruel to be kind.


Pete Rimmer23/05/2021 13:43:01
1121 forum posts
69 photos

I have a slide rule but I have no idea how to use it. Then again, I'm only in my early fifties

Nicholas Wheeler 123/05/2021 14:07:58
810 forum posts
59 photos
Posted by Pete Rimmer on 23/05/2021 13:43:01:

I have a slide rule but I have no idea how to use it. Then again, I'm only in my early fifties

When I took O-level maths in 1986, we all had calculators. But we were shown how to use slide rules as part of the reasons why logarithms are so useful.

HOWARDT23/05/2021 15:02:46
827 forum posts
28 photos

i used to work under a technical director, a doctor of engineering, who used to carry a six inch slide rule in his jacket pocket. Whenever he was asked a mathematical question, usually regrading hydraulics, he would go straight for his slide rule to perform the calculation. While he slid his rule I would perform a mental calculation and give him my rounded answer which he would eventually agree with.

Perhaps as with the discussion on Hydrogen central heating system we should move away from battery driven calculators and revert back to mental arithmetic with slide rules and log tables. Perhaps teaching maths without electronic aids is due a come back.

old mart23/05/2021 15:08:46
3485 forum posts
213 photos

I have one somewhere, they were quick to use, but you had to be able to know the approximate answer before using one as there was no decimal point. Log tables were more accurate, but when the electronic calculator came out, they were both finished. Even my cheap solar powered Casio's have 8 digits. When calculators were a new thing, I bought a desktop Sharp for £20 and was glad when my boss bought it off me for that much. It even had a percentage button and memory buttons.

SillyOldDuffer23/05/2021 15:28:10
7870 forum posts
1724 photos

Partial to a slide rule myself, and started a collection when I lived in a town with charity shops. Only got a couple of common examples before moving to the country put an end to it!

Slide rules are very useful whenever a match can be eyeballed; John's gear ratio example is a good one.

Before computers and mechanical calculating machines were available slide rules were often home-made by engineers to speed particular calculations. Being made of cardboard they rarely survive. These examples are from Albert Newby's 'Logarithmic Scales: Their Application to the Solution of Engineering and other Formulae : being a simple mechanical method of performing tedious calculations.', a paper submitted to the 1920-21 Session of the Association of Engineering and Shipbuilding Draughtsmen.

Simple formula example:


More complicated formula:




Three scales at once!


Rotary version of the second example.


It's quicker to set the maths up with a spreadsheet, but slides are faster to use when ranges of numbers are of interest and the job is done repeatedly. I find it easier to find suitable values on a scale than to scan tabulated numbers. Like analogue and digital displays, sometimes one form is more suitable than the other.

My main problem with slide rules is keeping track of the decimal point! My memory is terrible, making It necessary to write the numbers down in scientific form before attempting anything remotely complicated. No difference on a slide rule between 2 x 2, 20 x 200, or 2000 x 20000, so two sums are done, e.g.

2000 = 2.0x10³
20000 = 2.0x10⁴

Slide rule says 2.0 x 2.0 is 4.0 Paper calculation says 10³x10⁴ is 10⁷ so

4.0x10⁷, which is 40000000

Doing a sanity check to confirm the answer is the correct order of magnitude is pretty much essential. It's also necessary to keep a close eye on precision. Slide rules are normally only good for two digit precision, so 19.687 x 1960010 will be slightly wrong (1.97x10¹ x 1.96x10⁶ = 3.86x10⁷, not 38586716.870). For this reason I was schooled to call a Slide Rule a 'Fiddle Stick'.

Calculators have much better precision and remove the need for mental acrobatics and sanity checking.

On the other hand, answers flow remarkably quickly once a slide rule routine is established. I still use one occasionally. Log and Trig tables though, yuk!!! Nasty inaccurate and easily misread, at least by me.






Edited By SillyOldDuffer on 23/05/2021 15:28:46

Nicholas Wheeler 123/05/2021 15:44:15
810 forum posts
59 photos
Posted by old mart on 23/05/2021 15:08:46:

I have one somewhere, they were quick to use, but you had to be able to know the approximate answer before using one as there was no decimal point.

Knowing the approximate answer is necessary whenever the calculation is important.

Lots of people just copy the number off the calculator even when it's clearly wrong; 30 years ago my colleagues would hand in their banking claiming that five tills each with less than £1500 added up to £9k! More recently, my boss came out with a revised torque wrench setting for the adapter I'd made, and took a lot of convincing that it was wrong - her number had increased, when it needed to be smaller. There are lots of bolts where it probably wouldn't have mattered, but wing attachments are not the place to take chances....

Macolm23/05/2021 16:27:15
63 forum posts
12 photos

I have a 80mm diameter circular slide rule that I still use occasionally.


Nigel Graham 223/05/2021 16:36:38
1894 forum posts
26 photos

Oh it it all too easy to be convinced an absurd answer must be right because "the calculator / computer says... " , but you less often hear the old " computer error " excuse now because most people know the computer only does as it is told. Whether input error or programming error, it is still a human error, and in certain applications such as MS Access the computer reserves the right to tell you that you have cocked up, by simply crashing!


Reminds me of a conversation with one of my nephews one day, when he was in his teens.

He reckoned you don't need to learn much maths because "it's all in your calculator". He couldn't see why you still need know the mathemetics until I asked him how he would know which methods and numerical values in what order, to tell the device to use.

He was of course, unable to answer that.


Alan -

Your " I decided not to mention '.... log tables as I felt that would be a bit much for him!

How quickly is technology forgotten! "

Log tables as arithmetical tools, maybe, but certainly not Logarithms. They occur in many technical calculations - perhaps the most familiar example being the deciBel scale for sound, vibration and electrical signals, which is logarithmic to the base-10.

(E.g. purely spherical spreading loss of radiated sound pressure in dB = 20 log (distance); in metres, feet or the 'kiloyard' once used in the USA; provided all units are consistent throughout. )

Nick Clarke 323/05/2021 17:48:03
1314 forum posts
53 photos

And one of my most reread books is 'Slide Rule - The Autobiography of an Engineer' by Nevil Shute - It should be essential reading for any young engineer involved with a large project - and a good read for us older ones!

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