The Slide Rule · Halloran & Voss No. 4081

Under the hairlineThree figures · you keep the decimal point

Kx³: 1.00
Ax²: 1.00
Bslide: 1.00
CI1/x: 10.0
Cslide: 1.00
Dx: 1.00
Llog x: 0.000

Drag the slide
Drag the cursor
← → nudge · Shift fine

Instructions for useLeaflet 4081/A

Why sliding two sticks multiplies

log a + log b = log ab

A slide rule has no gears and no memory. It has two strips engraved so that the distance from 1 to any number is the logarithm of that number. That is why the graduations crowd together toward the right: the step from 1 to 2 is exactly as long as the step from 2 to 4, or from 4 to 8.

Lay one length after another and you have added them. Because log a + log b = log ab, the length for a followed by the length for b ends precisely at the mark for a × b. The stock carries the first length, the slide the second, and the hairline keeps your eye honest. Division is the same motion run backwards: take a length away.

The rule gives digits, never the decimal point; that part is yours. Three figures come easily, a fourth by judging between the ticks at the roomy left-hand end. For some three hundred and fifty years, until pocket calculators arrived in the 1970s, that was enough to size a beam, a boiler or a wing spar.

How to read a scale

The large numeral is the first digit. Between 1 and 2 the small numerals give the second, and every tick is one hundredth: the hairline above stands at 1.37. Further along a tick means more: two hundredths from 2 to 4, five hundredths from 4 to 10.

Keep the rule dry and the cursor clean
Halloran & Voss · Instrument Makers