Thanks to @chickenHeadKnob who graciously has allowed me to introduce my household to the slide rule. Now I have a few questions after reading the various manuals on calculating using a basic Sterling slide rule (having A, B, C1, C, D, K, S, T, L rules) and having to actually start using them in practice with my 10 year-old who was doing basic multiplication and division yesterday! By the way we also have a Soroban so I am trying to figure out how the Slide-Rule fits in with my strategy to work the brain muscles of my 3 kids.

Firstly, for doing multiplications of two 3-digit numbers, for example 761 x 423, I am using a Soroban to essentially add up the following:

3 (1 x 423)

20

400

180 (60 x 423)

1200

24000

2100 (700 x 423)

14000

280000

===== SUM

321903

Of course all this is done in the Soroban "on the fly" so you move beads around as you pair-off multiply the various digits so it is fairly fast but most importantly, it comes up with the exact answer. So I was going over this with my 10 year-old daughter and trying to show her how to use the slide-rule and I was puzzled as to how to get precise exact answers, if there is some "trick" or not. If they were used for calculations that were critical to have exact answers, how was that done?

Right now, to do the above multiplication I would set the numbers to scientific notation as 7.61 x 10^2 * 4.23 x 10^2. Then I would slide my "C" rule index 1 (the right side one) to match up with the 7.61 on the D scale, then wander along on the C until I see 4.23 and then read off the D scale. So I tried it out on this virtual slide rule:

http://www.antiquark.com/sliderule/sim/n909es/virtual-n909-es.htmlSo I did that as precisely as I could and ended up with 3.21 - 3.22. Of course, keeping in mind the 10^2 * 10^2, the answer ends up being 3.21-3.22 x 10^4 or around 321000 - 322000. This is approximately close to our exact answer of 321903. However, I had to make sure I lined up the "C" index 1 with 7.61... not exactly easy, and any error there would also misalign the 4.23 as well which would throw off reading the answer. And even when I did get an answer it would be only to maybe 3 digit precision, +/- on the last digit:

How am I supposed to get exact answers? Is there a way of "dividing and conquering" with this thing? Is it purely a quick estimation device? How could they land people on the moon using non-exact answers, or am I missing the point? More importantly... I am trying to teach my kids how to use and integrate the slide-rule into their homework and the math they are being asked to do wants exact answers!

Same goes for division... My daughter is learning simple long-division such as 3157 divided by 7 where we do this:

4 5 1

________

7 | 3 1 5 7

2 8

----

3 5

3 5

-------

0 7

7

- -----

0

So with the slide-rule, I would use the "C1" and "D" scales again as follows with 3.157 x 10^3 and 7... First I slide the 3.157 on "C1" to match up with "1" on the "D" scale so I am actually multiplying 1/3.157 with 7 on the "D" scale. I then slide cursor over to 7 on the "D" scale and I can see that my "C1" scale shows just barely over 4.5:

Again, keeping tabs on our powers we would have to divide out 10^3 (3.157) by 10^1 (of the 7) so we get 4.5 x 10^2 which is 450 or close to our answer but not exactly.

Can someone please let me know how (or if) I should be integrating a slide rule into these types of math problems the kids are getting at this point? If it is not possible to get the exact numbers they need for their test, is it just an easy way for them to check? Is there something else they can use it for? I am excited about using the Soroban and Slide-Rule with the kids and forgetting the calculator but I want to make sure I am using them in the appropriate ways.