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Last April I bought a slide rule.

It is a Post model 1447 made by Sun Hemmi in Japan. I found it in an antique store buried under a pile of other slide rules and slide rule instruction booklets. Its beautiful bamboo-and-brass construction caught my eye, and I had to get it. As a computer scientist and programmer, so much of what I build is nonphysicial, so I love the idea of holding a physical computing device.

That is why I was especially excited when last week I came across the following book in the library:

It is called An Easy Introduction to the Slide Rule. I found it completely by accident. Only after taking it off the shelf did I notice the most surprising feature of the book: it was written by Isaac Asimov, he of science fiction super-fame.

The book explains the slide rule starting from first principles. It is informative and very enjoyable to read, even though some of the motivating stories show their age. Here is an excerpt from the first chapter:

We have all heard, these days, of electronic computers. These marvelous instruments, which came into use during World War II, are capable of performing in a few seconds work that might take years if all we could use were pen and paper.

There are times when arithmetical problems come our way and we might wish that we ourselves owned such a computer to do the work for us. Such a situation would have its disadvantages, however. Electronic computers are bulky, expensive, complicated, and can be handled only by people with special training.

Besides, electronic computers aren't at their best when used for everyday problems. That would be like trying to shoot a fly with naval artillery.

For a fly, an ordinary swatter is much better, and for ordinary mathematical problems, we could best use a really simple computer.

There happens to be a simple computer, just suitable for everyday computations, that was invented about 350 years ago. It isn't electronic; there are no electric currents involved. In fact, it is no more than a piece of wood with some marks on it. It looks like a ruler except that it has a middle piece that can slide back and forth, so that it is called a slide rule.

If a 45 year old book on slide rules sounds interesting to you (and how couldn't it!?), then you might also like How Round Is Your Circle, which explains how one would actually make a slide rule using nothing but a pencil, a piece of string, and three pieces of wood.

Update Tuesday, August 17, 2010

While antique shopping, I came across this handmade (!), four-foot-long slide rule.

The tag says, "For the slide rule collector who has everything, this is a home made demonstration classroom slide rule. I've had others that were company produced, but this is the first [that was made by hand]."

I would love to meet whoever made it. I would ask how he or she machined the hardware and determined the positions of the markings.