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For more on marking an answer as the "Best Answer", please visit our FAQ.Calculators are infinitely more accurate than slide rules. Slide rules may give the larger units in a calculation, but smaller units become a matter of guess work.
Try the following - measure the length of an inch on a ruler by using the centimetre scale on another ruler. Do you get 2.54 cm? The .04 in the 2.54 becomes guesswork. Its the same problem with the marks on a slide rule.
The slide rule has been around a lot longer than the calculator, and has been largely kicked into touch because the use of a calculator requires little or no thought to get "the right answer". Slide rules will provide very quick answers, particularly for repetiitve clculations once set up, but the one thing they will not do is give you the position of the decimal point in the final number. (as Gnu says, they work on logarithms). I think that the occasional use of a slide rule will help to keep the brain active and force the operator to actually think about the calculation. One or two infamous space-related disasters may well have been caused by too much calculation and not enough thought. I make a point of keeping one in my desk and occasionally using it to confuse and intimidate the (much) younger engineers sitting around me. (But then, I am a fully qualified Grumpy old Engineer!)
If you want to explain the slide rule to a newcomer, start with two ordinary rulers and use them to add lengths together - eg, 2 cm on ruler 1 plus 3 cm on ruler 2 will arrive at 5 cm on ruler 1. Once this is in the brain (and, of course, subtraction), you will need to introduce powers of numbers and logarithms on a paper basis, finally revealing that if you multiply numbers by adding the logs, then you can multiply by adding the lengths of a logarithmically scaled ruler. Fait accompli!
Good luck.