Lab
Slide rule
A working interactive slide rule — the analog computer that got us to the moon. Drag the slide (middle section) left and right to set up a calculation, and drag the cursor hairline to read the result. Multiplication is addition of logarithms, made physical.
log(a × b) = log(a) + log(b) · drag slide and cursor to calculate
reading
Position cursor over a scale
Drag the middle slide left/right. Drag the red hairline independently.
1.0×
The slide rule was invented in the 1620s by William Oughtred, building on John Napier's recent invention of logarithms. The principle is simple: because log(a×b) = log(a) + log(b), you can multiply two numbers by physically adding two logarithmic distances. The C and D scales are the workhorses — align the 1 on C above a number on D, then read the product on D under any other number on C. Division is the reverse: align the divisor on C above the dividend on D, and read the quotient under the 1 on C.
The additional scales extend this into higher operations. The A and B scales are compressed to half the length, so they represent squares (reading from D to A gives you x²). The K scale compresses to a third, giving cubes. The CI scale runs in the opposite direction, giving reciprocals — reading CI against D directly computes division. A practiced engineer could chain these operations together to evaluate complex formulas in seconds.
Slide rules were the essential computing tool of science and engineering for 350 years. They designed the Empire State Building, the Golden Gate Bridge, and every Apollo mission. The HP-35 scientific calculator, released in 1972, ended their reign almost overnight. But for those three and a half centuries, all of engineering ran on two sliding pieces of wood and the deep insight that multiplication is just addition in disguise.