How to Use That Slide Rule
You have that slide rule in the back of the closet. Maybe it was from your college days. Maybe it was your Dad’s. Honestly. Do you know how to use it? Really? All the scales? That’s what we thought. [Amen Zwa, Esq.] not only tells you how slide rules came about, but also how to use many of the common scales. You can also see his collection and notes on being a casual slide rule collector and even a few maintenance tips.
The idea behind these computing devices is devilishly simple. It is well known that you can reduce a multiplication operation to addition if you have a table of logarithms. You simply take the log of both operands and add them. Then you do a reverse lookup in the table to get the answer.
For a simple example, you know the (base 10) log of 10 is 1 and the log of 1000 is 3. Adding those gives you 4, and, what do you know, 104 is 10,000, the correct answer. That’s easy when you are working with numbers like 10 and 1000 with base 10 logarithms, but it works with any base and with any wacky numbers you want to multiply.
The slide rule is essentially a log table on a stick. That’s how the most common scales work, at least. Many rules have other scales, so you can quickly, say, square or cube numbers (or find roots). Some specialized rules have scales for things like computing power.
We collect slide rules, too. Even oddball ones. We’ve often said that the barrier of learning to use a slide rule weeded out many bad engineers early.