## 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.
<p><em>&quot;Most slide rules include a C1 scale&quot; ... and in other part of text.</em></p><p>I think is not a C1 scale but a CI (inverted) scale.</p>
<p>You are correct. I misread the manual when I got my first slide rule more than 50 years ago and have been making that mistake consistently ever since. I have made corrections, also where I had D1 rather than DI.</p>
<p>Had to use a slide rule back in high school for a drafting class. Darn if I have forgotten how to use it. Great tool for when the power goes out. Kids of today do not know what they are missing.</p>
You are in a fortunate situation. Faber-Castell is a German company and you live in Germany, so you do not need to worry about arranging international shipping. It appears Faber-Castell is selling new old stock, that is, slide rules they produced before they ceased production in 1973, but never sold. On the one hand, these slide rules are almost 40 years old, but are also new because they have never been used. Thank you for your comment. I do not have a smart phone, but the slide rule application is interesting. There are also various other firms that sell slide rules, most of them used.
Yep, went all the way through college using a slide rule. Where I worked, the recent engineering hires had never seen one. After I graduated (1966) I took the first of my PE exams using a slide rule, but others brought those big mechanical/electrical calculating machines that were bigger than a typewriter. They had to arrive early to get a table near an electrical outlet. <br> <br>Bill
I almost wish I had gotten some training and experience using the log-log scales. As much as I enjoy using a slide rule, I have to find an excuse to do it. When a slide rule is nowhere near, I use the basic calculator on my cell phone. <br> <br>Thank you for looking. One day we old guys who can still use a slide rule will be appreciated.
Awesome. And I hope you go big, and go Christian for #200.
When I began submitting Instructables I thought I would run out of ideas after 25 or so. It was a shock to reach 50. But, things keep breaking around our house, or an idea comes to me that I want to share. Thanks.
...and to know that they have sent men to the moon with this thing!<br>Nice I'ble.
Thank you for looking and for your comment. The moon shots did include a primitive on-board electronic computer, but a huge amount of the engineering and any in flight human calculations would have used slide rules. In my sample problem of 259 x 653 the difference between the slide rule answer and the electronic calculator answer was 127 out of 169127. That is less than 1/10 of one percent. That is like calculating something to be at a distance of 1km with an error rate less than 1m. In a previous Instructable that I linked I summarized some points from a <a href="https://www.instructables.com/id/Refurbish-an-Old-Slide-Rule/" rel="nofollow">2006 article on the contribution of slide rules</a>&nbsp;(the final step). Things designed with a slide rule were a little over-engineered, but the slide rule was very effective. A final step in any engineering project was to check all calculations for mistakes.&nbsp;
Another great Ible from you my friend, thank you. <br>Also I would like to wish to you and your family a Happy Healthy and Prosperus New Year
Thank you, Steli. I wish you a prosperous and healthy New Year, too.
Phil, this is very valuable information, I want to go out and buy a slide rule.<br><br>I don't read all the text (it is late night). You can calculate also X**(2/3) and X**(3/2) with a cheap slide rule.
Thank you, Osvaldo. Ask around. Someone probably has a rule they would sell or give you. I thought you still had a slide rule.
Think Geek sells a slide rule that is very inexpensive. Bearing in mind that you get what you pay for it might be a good introduction. I had to take mine apart and clean it to make it slide anything like usable. At first I thought it was glued in place.
Thank you. I did link the ThinkGeek rule. But, because a number of reviewers had the difficulty you describe, I suggested interested people might want to check eBay for a comparable never sold plastic Pickett rule.
WOW. This looks interesting and complete. I have put this on a mental list of things to read and understand this month. <br><br>James
James, it is pretty complete, except for the material on logarithms and material related to placement of decimal points with square and cube roots, as well as material on square roots and cube roots of numbers less than 1. Had I digested and given all of that back in concise form this Instructable would have required considerably more steps. Over the years my most frequent uses of a slide rule have involved only a few squares or square roots and a very, very few cubes or cube roots. I think that could be true of many others, too. But, then my life's work does not involve a lot of mathematics. Still, I like to keep a slide rule next to where I sit while I am watching television. When a mathematical question comes into mind, I reach for that slide rule. Thank you for looking and for commenting.
I've never used one. I think I'll get one and learn.
If people know you are interested in slide rules, do not be surprised if someone comes to you with a slide rule that had belonged to someone in the family and the person who has it now would be very pleased to give it to you so it does not just gather dust. A nice Keuffel &amp; Esser rule from the late 1950s with all of the scales shown on the virtual slide rule in this Instructable can go for \$30 to \$50 US on eBay. I recently conducted a funeral for a man who had been an engineer. His widow knew from Facebook that I like slide rules. She gave me his K &amp; E Log-Log Trig slide rule. She was just happy it would be appreciated by someone. No one in the family had any interest in it. (I did find some corrosion had eaten the metal frames around the cursor and I spent about \$15 US restoring it. The corrosion also left discoloration spots on the white celluloid skin of the slide rule. I am hoping sunlight will bleach that out. Anyway, it is fun to solve a problem with a slide rule while your friends are still looking around to see who has a calculator.
I just downloaded an iPod app called <em>Cube Slide Rule Lite</em>, just to use these instructions.<br>
Thank you. I think that is just about the highest praise anyone could give.
:-)