Was there life before computer?

calculating instruments before the digital era

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Virtual slide rule

See how calculations used to be done before the days of electronic calculators, find out about an important piece of engineering history with this virtual slide rule: to perform the calculations click on the part you want to move an drag it with the mouse. Download here the printable template or the standalone program.

Before electronic hand held calculators, the slide rule was widely used in Engine- ering, Science and Commerce for rapidly performing calculations involving multipli- cation and division which have to be accurate to not more than three or four decimal places. This model is really basic and very easy to use.

Due to the size of the slide rule the page is intentionally out of the frame.

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Short course

Example: 2.3 x 3.4 (uses C and D scales)

  • slide the C leftmost '1' on by side 2.3 on the D scale;
  • move the cursor by side 3.4 on the C scale;
  • the cursor is now on the D scale just a bit over 7.8. The correct answer is 7.82.

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Example: 4.5 / 7.8 (uses C and D scales)

  • move the cursor by side 4.5 on the D scale;
  • slide 7.8 on the C scale by side the cursor;
  • the C rightmost '1' is now at 5.76 on the D scale. We know that the result of 4/8 is near 0.5, so we adjust the decimal place to get 0.576. The correct answer is 0.576.

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Example: 2.5 2 (uses A and D or B and C scales)

  • moving the cursor by side 2.5 on the C scale; we get on the B scale ca. 6.25; The correct answer is 6,25.

Example: √3.500

  • the A and B scales have two similar halves. The left half is used to find the square root of numbers with odd numbers of digits; the right half is used for numbers with even numbers of digits. Since 3.500 has an even number of digits we'll use the right half of the scale. Moving the cursor by side 3,5 of the A/D scales we get on the C/D scales ca. 59,15. The correct answer is 59,16.

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Now we can try this operation: √350 / 1,51

  • moving the cursor by side the 350 of the A scale (odd number of digits, then the left side) we get its square root, 18.7, on the D scale;
  • now we match 18.7 with 1.51 of the C scale: on the D scale, in correspondence with the C leftmost index '1', we can read the answer: ca. 12.35.

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Not bad in a couple of seconds, an electronic calculator would have been just a little more precise, finding 12.3896. This slight approximation has not prevented von Braun to send the Man on the Moon: the slide rule is in fact less difficult than it sounds, the secret is just to practice, to practice and to practice ...

Nicola Marras 2008


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