Wow!  What do those words mean?

They sound very complicated, but they secretly have a simple meaning.  Really, these words can be used to describe the same thing but it's probably best that you know both words.

Let's look at an example first ...

 Hmmm ... that looks right, so what are we talking about?

The fraction one-fifteenth is given a special name here.  it's called the reciprocal of 15 or the inverse of 15. (Actually, the correct mathematical term is the multiplicative inverse.  Now that should impress someone!)

Now as far as we're concerned there is very little difference between the two words when working with slide rules, in fact, we generally use the word inverse.  In our example the fraction, one-fifteenth is the inverse of 15 because it does the opposite to something.  Can you guess what it is?

That's it!  The division by 15 reverses the multiplication by 15.  Division is said to be the inverse of multiplication.  So in our example we have 15 multiplied by its inverse (we divide by 15 or multiply by one-fifteenth) and the operation is undone ... we end up with 1.

What about reciprocal?

Well 15 can be written as

If we turn this upside down, we get

So we can think of a reciprocal as just turning the fraction upside down.

"Okay", I hear you say, "but what's that got to do with a slide rule?"

Well a slide rule has a special scale on it called the CI scale and normally it is written in red.  It is actually an inverse C scale and we read this scale from right to left.  If you have a close look at the CI scale you will see the numbers getting larger as you go from right to left, whereas the C scale gets larger going from left to right.

After all that, let's have a brief look at how we use this CI scale.