[R-390] was: Theory Question now: abacus (OT)

[Bill Hawkins]

A slide rule works on the principle of adding two numbers by sliding
one identical scale against another. If you have two 12" rulers, you
can add 4 + 2 by sliding the second ruler so that zero inches on it
is opposite 4" on the other. Then go up 2" on the sliding ruler and
read 6" on the stationary ruler. Works for adding fractions, too.

The reason the slide rule works for multiply and divide is a property
of logarithms. To multiply, you add the algorithms of the two numbers.
Subtracting the log of the denominator from the log of the numerator
give the answer for division. You don't need tables of logs if the
scale on the rulers is logarithmic.

Put another way, you do it just like you did on an Ohmite resistance
slide rule.

Bill Hawkins


-----Original Message-----
From: Dennis Wade
Sent: Thursday, December 04, 2008 11:31 AM

Well, I was taught in the early '70s as a high school vocational
student.
I LOVED my yellow Pickett slide rule...and even did logs.
Trouble is...I'd need to be re-taught now.  I don't think many more
classes after mine were taught slide rule techniques.

             Dennis