diagram node · 2026-06-06
diagram voice-memo public

# The Slide Rule

Drag the slide. Multiplication becomes sliding two log-spaced rulers past each other — adding lengths that happen to be logarithms. Line up the strip's 2 under the top 4, and the strip's 4 is just sitting there under the top 8. Nothing computes; the wood and your eye do. This page is a jumping-off point for a live conversation about how the thing actually works.

No calculator here — the alignment IS the answer. This is a faithful physical slide rule: nothing computes for you, nothing reads out a number. To do 2 × 4, grab the middle strip and slide its 2 directly under the top scale's 4. Now look at the strip's 4 — it has come to rest exactly under the top scale's 8. You did the multiplication with your hand; you read the result with your eye. (Try it. The red glass cursor just helps you line your eye up — it doesn't tell you anything.)
D · fixed
C · slides ⟷
D · fixed
line up:

This is a real slide rule — it computes nothing for you. Grab the middle C strip and drag it so its 2 sits directly under D's 4. Then look down the strip to C's 4: it now rests under D's 8. That is 2 × 4 = 8 — you read it straight off the wood. Slide the red glass cursor to help your eye hop between scales. (The buttons just pre-slide the strip for you; the answer is always wherever the numbers line up.)

## Why sliding = multiplying

1 · Log spacing
Each scale is laid out so the distance from the left edge to a number is its logarithm. 1 sits at 0, 10 sits at one full decade. The numbers crowd together as they climb — that crowding is the log curve.
2 · Sliding adds lengths
Slide C so its 2 sits over 4 on D — you've shifted everything right by the length log(4)−log(2). Now C's 4 sits at that shift + log(4) = log(8). The strip's number landed on D's 8 on its own.
3 · The label IS the answer
log(2)+log(4) = log(8). So the D number printed under that point is literally 8 — no computation, no readout. The wood did the addition; the engraved scale did the exponential lookup for free. You just read it.

Move on the rule | What it means in logs
--- | ---
Slide C's 2 under D's 4 | the whole C scale shifts right by log(4)−log(2) = log(2)
Look at C's 4 | it now sits at log(4) + log(2) on D
Read the D number under it → 8 | log(4)+log(2)=log(8), and the printed D label is 8. That's 2 × 4.
Same setting also gives 3 × 2, 5 × 2, … | once C's 2 is under D's 4 the strip is "× 2" everywhere — read any C value against D

## Real interactive slide rules people have built

- sliderules.org — virtual slide rule · drag a faithful C/D/A/K/L scale set in the browser
- Antiquark Slide Rule Simulator · the classic Flash-era sim, now JS — clean and draggable
- SRC Slide Rule Museum simulator · a richly-scaled Pickett-style rule
- Wikipedia: Slide rule · scale-by-scale reference (C, D, A, K, S, T, L…)

If any link has rotted, search "virtual slide rule" / "slide rule simulator" — there are many.

## Jumping-off questions for the live conversation

- Why does the answer "wrap around" past 10, and what's the off-scale trick (slide the 10 instead of the 1)?
- What are the A and K scales (squares, cubes) and why are they just compressed copies of D?
- How do the S, T, L scales fold trig and plain logs onto the same stick?
- Why did this die the day the pocket calculator shipped — and what was lost (the felt sense of magnitude)?

Related: [[voice-memo]]

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