# [SI-LIST] Re: Slide Rules. (Was Re: We ought to address mutual SI-EMI issues in this forum)

• From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxx>
• To: "Michael Smith" <michael@xxxxxxxxxx>, <kimgh@xxxxxxxxx>
• Date: Fri, 13 Sep 2002 12:02:55 -0600

```All - especially Roy,
I said that tounge-in-cheek just to have a little fun.  I'm as old? as the
rest of you geezers, and even have the circular slide rule mentioned below.
My wife's father gave it to me.  If you've never seen one, they are really
cool.   I actually used it in class for a week or so(with an HP backup for
tests)when I was in college.  You can imagine the interest it engendered.
I especially enjoyed Kim H.'s response.
Ken

-----Original Message-----
From: Michael Smith [mailto:michael@xxxxxxxxxx]
Sent: Friday, September 13, 2002 10:37 AM
To: kimgh@xxxxxxxxx; Ken.Cantrell@xxxxxxxxxxx
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Slide Rules. (Was Re: We ought to address mutual
SI-EMI issues in this forum)

To keep the fun slide rule thread going...

The mystery of the slide rule was based on the fact that if Z = X * Y
then log Z = log X + log Y.  Therefore if you had a two log scales which
could slide against one another, you could measure the addition of the
logarithms of two numbers and the result would be proportional to the
logarithm of the multiplication of the original two numbers.  Other math
functions like division could be derived using logarithms in a similar
way.  If you match the lengths of the two logarithms being added to the
required resultant logarithm then you have performed a square root....

Slide rules in a rotating circular scale form are still used today by
student pilots.  They are used to quickly perform speed/time/distance as
well as fuel calculations in the cockpit during cross country flights.
These "flight computers" are guaranteed never to run out of batteries
and they never have signal integrity issues in any weather conditions.

And now back to our regularly scheduled signal integrity issues.

Cheers,

Michael Smith
Hardware Engineer
iZ Technology Corp.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Kim Helliwell
Sent: Friday, September 13, 2002 8:47 AM
To: Ken.Cantrell@xxxxxxxxxxx
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Slide Rules. (Was Re: We ought to address mutual
SI-EMI issues in this forum)

On Friday, September 13, 2002, at 07:08  AM, Ken Cantrell wrote:

>
> Roy,
> Excellent summary and opinion, I'm sure all agree.  I've got just one
> question:
> what is a slide rule?

Hmmmm, how old are you, sonny?

A slide rule was an archaic device, made of bamboo,
plastic, glass, and metal. It's what passed for a calculator
in my college days. Geeks (then known as "eggheads") of my
day wore them on their belts, much as they later
wore their calculator holsters.

Slide rules could not do addition; that was presumed
to be something a human could still do in those days.
But they could multiply, divide, take logs, do square and cube
roots, calculate trig functions and exponentials, etc.

Calculations were done by manipulating a center slide,
lining up marks ruled on the slide with marks on the
stationary portion; then lining up a transparent cursor
with a ruled line over other marks to read off the
results of a calculation. How the slide rule enabled
one to make such calculations is still something of a
mystery; results were seldom very accurate. Depending
on where on the scale your calculation was located,
you could achieve accuracies only of just over 2 decimal
places to (nearly) four places. But often the last place
was mostly a matter of squinting and guesswork.

Slide rules required no batteries or other "power"
source, other than the hands required to push the
slide and cursor around. There were no buttons
or displays required. In fact, it seems incomprehensible
that such a thing could ever have existed, let alone
been used for hundreds of years.

Today, sightings of slide rules are rare. They might
be found in museums, in the ruins of old aerospace
buildings, or forgotten at the bottom of a grizzled
old engineer's drawer.  Slide rules once ruled, but
have been supplanted by battery powered electronic
on....

Big :-), just in case!

>
> Ken

Kim Helliwell
Apple Computer
kimgh@xxxxxxxxx
408 974 9936

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