Multiplication is a very simple operation to perform with a slide rule.  To do this, we make use of the C and D scales.

The way we do this is to line up one of the indices of the C scale with the first factor on the D scale.  Let's look at a simple example:

2 x 3

Yes, I know ... you probably know what 2 x 3 is but these simple examples illustrate very easily what is going on and you can find the answer, so let's go!

2 is the first factor, so move the slider so that the left index (the left 1 on the C scale) is above the large number 2 on the D scale.

So your slider should be to the right.  Now this picture is a little hard to read, so we'll zoom in on part of it ...

Now straight away, you can probably work out how to find the answer!

Did you find the answer without any trouble?  Excellent!

Now what did we do here? 

1. We line up the left index of C (we call this C1) with the first number (on the D scale).
2. Then we move the cursor to the second number on the C scale.
3. And finally we read the answer below the second number (from the D scale).

Let's have another look at our slide rule ...

Do you see the new names we give to the numbers?  We actually use their correct names but we put the scale name in front of them so that anyone who uses a slide rule will know what we are talking about (and it's certainly going to impress other people).  This time I've drawn in the hairline of the cursor in red ... See how it passes over the 3 and 6?

Are you ready to have go at one?  Read over the three steps again and then try this one:

2.5 x 3

When you think you've got the answer click here to check it and when you're ready come back.

Ready? Let's try another one!

Okay, let's see how good you really are!

Our next practice problem is ... Are you ready for it?

3.4 x 2.5

Right!  Off you go and then click here when you think you've got the answer.  We'll see if you got it right!

Well that was pretty easy wasn't it?  But sometimes our problems don't always work out the way we want them to.  I'll show you what I mean, let's try the problem

4.8 x 5.5

Now when I move C1 to D4.8 watch what happens!

You try it ... 5.5 is way off the scale and you can't read anything from the D scale!

Does this mean our slide rule is no good?  Never!

This is when we start using the other index or the right index of the C scale.  The right index can also be called C1, but sometime it is called C10.  I'll call it C10 so that I don't confuse you.

When this sort of thing happens, we move the slider so that C10 is above the first number, then we move the cursor to the second number on the C scale and read our answer underneath on the D scale.  Almost the same as before.  So to do this problem:

1. Move the slider so that C10 above D4.8
2. Move the cursor to C5.5
3. Read the answer under C5.5 which is D26.4

Let's go over those steps again!

Whenever the second number goes off scale we ...

1. Line up the right index of the C scale (C10) with the first number (on the D scale).
2. Then we move the cursor to the second number on the C scale.
3. And finally, we read our answer below the second number (on the D scale).

Now before we go on ... How did I know the answer to the problem was 26.4?

This is one of the tricky things with a slide rule!  Although it can give me the right answer, sometimes when I read the scale, it won't always tell me where the decimal point should go.  If you have a look in the picture above, it looks like 2.64 not 26.4.  This is where we have to use a bit of math or guestimation!  The two numbers were 4.8 and 5.5; now I could round these and say they're about 5 and 6 so I know my answer should be around 30 when I multiply them.  That's how I knew the answer must be 26.4 and not 2.64 ... you'll learn more about this sort of thing later.

Okay ... Are you ready to try one?

6.5 x 2.8

When you think you've got it, click here to check your answer!

Well, I think it's time for some practice questions!  What do you think?

Off you go, click here ... Good luck!