Log tables
Slide rule with a log scale?
Log tables
TBH I'd have thought that the ease 2.5mm²7X goes into a blue crimp [and 2x 1.5mm² tri-rated usually fits] that 7/0.029" would also fit. That's D=2.045mm/80.5thou Vs 2.21mm/87thou.I used Knotts log tables on Castles, and yes the slide rule did have a log scale, but significant figures much reduced, today I use the go/no go gauge i.e. will a blue terminal fit. I would guess a 2.5 mm² is around 0.070 inches? but in real terms this is the problem, have I made a mistake? Is 89 thou 4 mm²? or 6 mm² 109 thou? I have a micrometer but I don't know off hand what the reading should be, so easy to forget it's radius not diameter and end up out be a factor of 2. OK today with excel or java script you can make a converter. But in the main we don't use the micrometer we use red, blue, yellow crimps as see which fits. This means it is all too easy to think a 2.9 mm² cable is 4 mm² it will not fit in blue, and slack in yellow so must be 4 mm², that is only true if cable is metric.
Indeed - and that is remarkably close to the 'true' answer. It assumes that pi = 22/7, and that's almost true - pi is actually 3.1415925..... and 122/7 is 3.1428571 (recurring) - so the approximation is accurate to within about 0.04%.The simple formula for 7 strands is 22r² or 5.5d² to find the area.
There isn't a difference - you've just said the same thing in two different ways - if 7 x pi is 'near enough 22", then pi is also 'near enough' to 22/7Not sure that there is any difference but - rather than assuming that pi is 22/7, surely it realises that 7 x pi is near enough 22, namely 21.99(11485etc.).
Quite possibly, but so what?Agreed, but surely people didn't begin by dividing 22 by 7 and seeing what the answer was; they would have multiplied pi by 7 because that is the number of strands.
As I implied, I can't read what was in 'their' minds.Ok, but I just don't think they discovered the 22/7 by doing it that way round - unless you think they might have tried 23/7 first and found that it was too much.
pi is actually 3.14159265...pi is actually 3.1415925....
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