===========================
Log-scale calculation tools
===========================
:CreationDate: 2011-05-23 18:46:11
:Id: HW/logscale-calc-tools
:tags: - hardware
- vintage
Uh? What?
=========
Maybe you know better the name "slide rules". Or maybe not even that
one… Short version: ``log(x*y) = log(x)+log(y)``, so if you have a
pair of rulers with a logarithmic scale, you can use them to perform
multiplications and divisions. More details on dedicated sites, like
`Ron Manley's one <http://www.sliderule.ca/>`_ o `Eric Marcotte's one
<http://www.sliderules.info/>`_.
All right, let's pretend we've understood that…
===============================================
I have here a classic slide rule, and a cylindrical slide "rule" (of
the `Otis King <http://en.wikipedia.org/wiki/Otis_King>`_ variety). I
want to show how to perform a couple of simple operations.
Multiplication
==============
We want to multiply 7 by 3. Easy, right?
Let's start with the slide rule:
1) make 1 on the C scale match with 7 on the D scale
.. image:: s7x3-1.jpg
:alt: 1 on C scale matching with 7 on D scale
2) look with what, on the D scale, matches 3 on the C scale
.. image:: s7x3-2.jpg
:alt: 3 on C scale matching with 21 on D scale
21, which is our result
With the cylindrical rule things are a bit more awkward.
1) point to 1 on the lower scale, with the cursor
.. image:: r7x3-2.jpg
:alt: cursor pointing to 1 on lower scale
2) point 7 on the upper scale, *moving only this scale*
.. image:: r7x3-1.jpg
:alt: cursor pointing to 7 on upper scale
3) now move *only the cursor*, making it point to 3 on the lower scale
.. image:: r7x3-3.jpg
:alt: cursor pointing to 3 on lower scale
4) on the upper scale we read our result
.. image:: r7x3-4.jpg
:alt: cursor pointing to 21 on upper scale
It's totally equivalent to the linear rule: the two logarithmic scales
are equal, so a translation on one, "sums" to the other.
Division
========
Let's divide 24 by 4.
Linear rule:
1) make 4 on the C scale match with 24 on the D scale
.. image:: s24d4-1.jpg
:alt: 4 on C scale matching with 24 on D scale
2) look with what, on the D scale, matches 1 on the C scale
.. image:: s24d4-2.jpg
:alt: 1 on C scale matching 6 on D scale
6, again our result.
Cylindrical rule:
1) point to 4 on the lower scale, with the cursor
.. image:: r24d4-2.jpg
:alt: cursor pointing to 4 on lower scale
2) point 24 on the upper scale, *moving only this scale*
.. image:: r24d4-1.jpg
:alt: cursor pointing to 24 on upper scale
3) now move *only the cursor*, making it point to 1 on the lower scale
.. image:: r24d4-3.jpg
:alt: cursor pointing to 1 on lower scale
4) on the upper scale we read our result
.. image:: r24d4-4.jpg
:alt: cursor pointing to 6 on upper scale
Some notes
==========
The advantage of the cylindrical rule is that it allows longer scales
in a small space; the longer the scale, the more precise you can be in
reading it.
The advantages of the linear rule are in the ease of use, and in the
simplicity of concatenating operations one after the one, especially
when using more than 2 scales (the slide rule you see above has 6
scales: two regular ones, two quadratic, one reciprocal, one cubic).