Re: Concise Circular No 28

Matthew Lecin

>355/113 is a much closer approximation than 22/7 to pi, but neither are exact

355/113 is good to 6 decimal places! If you need something more precise you're calculating what? The great circle route around the Earth?


On Sun, May 24, 2020, 13:14 Andreas Poschinger <andreas.poschinger@...> wrote:

This remembers me in the "logarithmic uncertainty principle" (Logarithmische Unschärfe) which a friend told me who worked in chemical industry at slide rule times.

I'd say according to logarithmic uncertainty 22/7 is a good enough approximation on the slide rule with the advantage of for 22 and 7 really existing ticks (our other discussion...) - thus these values can be found without uncertainty.

Best regards


Am 24.05.2020 um 18:05 schrieb Bjørn Kisbye Engsig via
We shouldn't forget that some of these integer ratios are indeed exact, while others aren't. 127/5 is exactly 25.4 (in to mm) but 32/35 is not exactly 0.9144
(yard to m). This is of course the beauty of (in particular circular) slide rules where integers may be easier to remember. 

I do also recall that 355/113 is a much closer approximation than 22/7 to pi, but neither are exact. 

I own another circular slide rule with ratios mentioned, namely those used by the graphics and printing industry. 


Sent from my Sinclair ZX81

fre. 22. maj 2020 07.35 skrev Not Here via <>:
Very Interesting I never noticed that, thanks for the information.


On Thursday, May 21, 2020, 4:31:06 AM PDT, Markjsr <mj0760@...> wrote:

I was messing around with my Concise No. 28 Circular rule recently.  

The back has 38 different unit conversions in pie wedges. I always thought that was cool enough.

But we just discovered that most of the conversion wedges have a code indicating how to set the rule.

For example, to convert Miles to Kilometers There are 1.6093 kilometers per mile. The wedge says
"87 opp 140". When you set 87 on the inner C scale opposite 140 on the D outer scale, you get
1.6093 kilometers per mile. Once it is set, you can read miles on the C scale and get kilometers or
vice versa...for any value of miles or kilometers. Same for most of the other conversions. The OPP
settings are integer readings on both scales, to improve accuracy.  

Yes, you could just index one scale to 1.6093 on the other, but that would involve estimation and
would result in imprecise readings.  Integer values minimize guesswork.

Once the ratio between one unit and the other is set, you just read off the values.  

This makes me curious if other rules with unit conversion cards include optimal (integer) settings
for conversion.

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