Slide the scales and read the cursor to do multiplication
Before electronic calculators became common in the 1970s, scientists and engineers used slide rules like this for multiplication and division. The secret is logarithms — they use the principle of “turning multiplication into addition of logarithms.” Slide the lower scale and move the red cursor to calculate a multiplication yourself.
Principle: log(a×b) = log(a) + log(b). If you make a ruler with numbers placed at their log-value positions, multiplication can be calculated simply by placing the two scales side by side and adding lengths. ① Slide the lower scale so that “1” on the upper scale lines up with your desired number (a), then ② move the red cursor to point to another number (b) on the lower scale. The number the cursor points to on the upper scale is a×b.
① Slide the lower scalea = 1.00
🎯 Goal: Align the scale and cursor so the result becomes -
② Drag the red vertical line (cursor) left and right
💡 The gray section is where the result falls outside the scale (beyond 10), so it cannot be read on this setting. Real slide rules handle this by sliding the scale again.