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A More Complete Slide Rule Tutorial

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Step 5: Multiplying 2 x 2 and getting 4 as the answer

Now it is time to do an actual multiplication problem on the virtual slide rule. You can replicate this by going to the link for the virtual slide rule in the Introduction and following the steps yourself. Move 1 on the C scale until it rests directly over 2 on the D scale. Move the cursor until the hairline comes to rest over 2 on the C scale. The answer will be on the D scale directly below the 2 on the C scale. The answer is 4. (In the past when I would teach someone how to use a slide rule, I described this as a lazy "Z" pattern. It is actually an inverted "Z" laid over on its side. You move downward from 1 on the C scale to the first factor on the D scale, then upward and across to the other factor on the C scale. Finally move downward to the answer. The series of movements forms an inverted "Z.")

Multiplication on a slide rule means adding the physical length associated with one factor to that of the second factor and reading the product directly below the second factor. More explanation of how and why this works will be offered later when logarithms are discussed.

Just for fun, look at the graphic again. Find 1.5 on the C scale. Notice 3 below it on the D scale. 2 x 1.5 = 3. Find 2.5 on the C scale. Notice the 5 below it on the D scale. 2 x 2.5 = 5. The slide rule can be very handy when you must multiply a series of numbers by the same factor. Simply move the hairline down the rule and the answers are already in place under each of the other factors. It is a little like entering one factor into the memory of an electronic calculator and pressing memory recall for each of the other factors in the series, except that there are fewer steps involved with the slide rule. This is an example of a problem that can be solved more quickly on the slide rule than on a calculator.  With practice many people find they are as fast or faster on a slide rule than they are on a calculator.

Earlier I mentioned checking a slide rule for accuracy. I like to multiply 2 by a series of numbers, like 2, 2.5, 3, 3.5, 4, 4.5, and 5. The appropriate lines on the C and D scales should align exactly all of the way across, if the slide rule is accurate.
 
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