Peter Todd wrote:

>>> I made a bamboo slide rule for my dad last christmas, figured it was
>>> fair as I have his one at my apartment. Still gotta learn how to use
>>> it though.
>> 
>> Do it. takes about zero time to get the basics. Some advanced stuff 
>> takes more effort or is easily forgotten. Being able to do simple 
>> multiplication and division with pieces of wood or plastic gives 
>> increased appreciation of the shoulders we are standing on.
> 
> Also been told some people like sliderules because it shows you the
> relative magnitudes of the numbers you are working on at all times.

This is not quite correct. The slide rule does /not/ show you the magnitude
of the numbers, it only shows you the number itself; no decimal point, so
to speak. (Or better said, while working on the slide rule, the decimal
point is in an arbitrary position. Arbitrary WRT the actual calculation,
that is.)

But the crucial thing is that when working with a slide rule, /you/ have to
do the magnitude calculation and put the decimal point in the right place.
For example, you estimate the result to be something between 200 and 500.
The slide rule may give you the number 3.87, and by combining your estimate
of the magnitude with the numeric sequence from the slide rule you know the
result is 387.

And this is the part that sadly got largely lost with electronic
calculators. Some people who have that inclination very strongly still know
it anyway. For some this is so strange that they wouldn't learn it even
when working with slide rules. But for the many in between, not inclined
enough to know it anyway and not too estranged to it so that they never
would have learned it, they miss out on something that is IMO a valuable
skill: knowing how to estimate the magnitude of a numeric result just by
looking at the numbers.

> Aparently the calculations were done on an electronic calculator, and he
> person doing them made a small error with the decimil points... 

That's exactly the thing I was writing about.

> Ahh, yeah, I remember the concept from highschool math, where even in my
> time the teacher mentioned slide rules while teaching logorithms. I've
> yet to actually get the details correct. 

y = a * b;
log y = log a + log b;

That's basically it. On a slide rule, you have the numbers placed on
locations that correspond to the logarithm of the number. Say you have a 10
cm slide rule. log 1 = 0, so 1 is at the start of the scale. log 10 = 1, so
10 is at the end of the scale. log 2 = 0.30, so 2 is at 3 cm. log 3 = 0.48,
so 3 is at 4.8 cm. 

Adding 3 cm (2) to 4.8 cm (3) gives 7.8 cm. This is exactly where the 6 is,
because log 6 = 0.78, so the 6 is at 7.8 cm.

The only additional step is that when adding 7.8 cm (6) to 3 cm (2), this
results in 10.8 cm, which is beyond the 10 cm we had said our slide rule
has. But since we disregard decimal point positions anyway, this just wraps
over to 0.8 cm, by subtracting 10 cm (which is a factor of 10, or in other
words just a matter of the irrelevant position of the decimal point). And
0.8 cm is where 1.2 is on the rule, because log 1.2 = 0.08, or 0.8 cm with
a 10 cm scale.

Gerhard

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