11 Jul
2012
11 Jul
'12
1:21 p.m.
We're about the same age, I think, and in the
early 1970s I was taught
both imperial and metric. I use a mix of both in my head - lengths in
As I still do. My lathe is calibrated in inches (and 'thou' == 1/1000"),
but I use metric twist drills and metric fasteners. For accurate work
like that, I use my HP calculator which has the very useful 'unit
management'.
Incidentally, it is a great annoyance to machinists everywhere that there
are 2.5.4mm in an inch. Point being that 254 factorises as 2*127 (127 is
prime). SO do do exact metric screwcutting on an imperial lathe or vice
versa you need a 127 tooth changewheel.
However, there is a little trick...
8000/(3*21) is very close to 127 (in fact 8001/(3*127) _is_ 127). Since
8000 factorises easily into small numbers, you can get way with a 21
tooth changewheel as the only unusual one (all the otehr factsrs are easy
to get from normal changewheels) for non-critical work. It's OK for
making fasteners, etc, but not for making a micrometer screw :-)).
You really didn't want to know that, right :-)
feet make more intuitive sense than metres, distances
in miles and
speeds in mph rather than km or kph, for instance, although I'm quite
happy in both. I can't remember how many yards in a mile or ounces in
I can rememerb all fo them. Mainly becasue a lot of the numbers come up
elsewhwere.
Thejre are 1760 yeards in a mile. 1760 is a numebr that is etched into my
brain. Any idea waht? Well, it's 16*110, and most UARTs (serial chips)
need a clock that's 16 times rthe baud rate). So 1760Hz is the clock
freqeucny for 110 baud, as needed for taling to ASR33s.
But there si a more practical point here. Although miles and yards are
both units of length/distance, it's very rare fro the average pserson to
need to convert between them. You use yards (feet and inches too) to
measure your room for carpet or wallpaper. You use miles to work out how
far it is to go to your relaitves (or wahtever). You don't have to relate
them. Similarly you use poudns and ounces in the kitchen, stones when
figuring out if you've eaten too much from said kitchen, and tons when
you have to lift the family car (or whatever). Again, you don't need to
relate them.
a pound or pounds in a stone or any of that rubbish
though, so I use
metric for all those sorts of things unless it's a direct comparison -
e.g. British male clothes are sized in inches so I just go with the
flow. I have a 38" waist and a 36" inside leg because that's what all
the shops sell. OTOH I'm 6'2" tall or 1.88m depending on who's asking
- either is fine.
I was once anked on a job applciation form for my 'weight'. I haev no
idea why they wanted ot know this, I can't see how it can have any
relevance to my abilities as an electronic designer. But I gave it anyway
-- in Newtons. If they ask a scientist for a weight, it's going to come
in units of weight (or force). If they want mass, they should ask for it :-)
But Fahrenheit never made any sense to me at all. One
end of the scale
is entirely arbitrary, the other is an entirely different measure and
one that they measured wrongly anyway. It's seemed stupid and random
It's not totally claear why fixed points were originally used. One
version I heard went like this :
He wanted all temperatures to be +ve, so he did indded make the coldest
ice.salt mixture he could nad used that as a check (but not as the 0 of
his scale)
Melting ice was 32. Body temperature was 96. Point being there are then
64 degrees betwwen those points which is very easy to divide alogn a
thermometer tube (dbisecting is a lot easier than dviding into tenths).
and senseless since I was a small child and I've
never ever used it
for anything. Celsius is obvious and logical and rational. 0=B0 is
freezing, 100=B0 is boiling, bosh, there you are.
Nto to me it isn't. Why pick water anyway? And waht is this insane love
of multiples/powers of 10?
I much prefer an absolute temperature scale. I normally use kelvin...
-tony