CAMBRIDGE · 1949 · MERCURY ACOUSTIC STORE
DELAYLINE A number that exists only as sound in a metre of mercury — memory you must keep catching.
In the Mathematical Laboratory at Cambridge, EDSAC's numbers were not written anywhere. A 17-digit binary word was a train of ultrasonic pulses swimming down a steel tube of mercury — detected at the far end, squared up by a valve amplifier, and fired back in before anything could be lost. The word below is live. It is circulating right now, and at no instant does it exist all at once.
▼ THE STORE IS RUNNING — each register lamp lights only as its pulse passes the read tap
Its word is still safe — regenerated some 1,800 times a second, in principle.
- LAP
- 0
- CIRCULATING WORD
- 1949
- PULSES ALIVE
- 8 / 17
- MERCURY SWUM
- 0 m at tank scale
01 · THE TANK
Five feet of warm mercury, and a crystal at each end
At the transmitting end sits a quartz plate, cut so it rings at ultrasonic rates. Pulse it with voltage and it flexes, tapping a pressure wave into the metal: a binary 1 is a burst of sound, a 0 is deliberate silence in that time slot. The burst swims the length of the tube at roughly 1,450 metres a second — sluggish, next to electronics, and that sluggishness is the entire invention. A metre and a half of mercury holds around a millisecond of sound in flight, and a millisecond is room for hundreds of pulses at once.
At the far end a second crystal feels each compression arrive and turns it back into a whisper of voltage — attenuated by the mercury, smeared by dispersion, but still legibly a pulse. Amplify it, reshape it, fire it back into the transmitting crystal, and the word chases its own tail forever. Nothing is ever stored. It is only ever caught — thousands of times a second, each catch a small act of faith in the amplifier.
- 01TAPThe quartz transducer converts a voltage pulse into a pressure wave in the mercury.
- 02SWIMThe packet crosses the tank, losing height to absorption and sharpness to dispersion.
- 03DETECTThe receiving crystal reads what is left — in this tank, 0.72 of the injected amplitude, times each crystal's coupling.
- 04RESHAPEA discriminator judges the survivor against a 0.50 threshold and squares it back toward full height.
- 05RE-INJECTA clean pulse re-enters the tube. One lap is complete; the word has survived another audit.
02 · REGENERATION
The amplifier is the whole trick
Watch the small scope on the amplifier chassis under the tube. The IN trace is what the receiving crystal actually hears: a pulse shrunk to about seven-tenths of its height and smeared wider than it was sent. The OUT trace is what the reshaper returns to the tube: full height, square shoulders. That restoration, lap after lap, is the memory. Mercury only ever forgets; the valves do all the remembering.
This simulation draws the recurrence honestly rather than miming it. Each pulse carries a real amplitude. Every arrival is computed, judged, and either squared up, partly restored, or lost — and the register above the tube can only echo what the discriminator reports. Slide the gain and you are moving a real term in a real equation, not a style knob.
arrival ≥ 0.70 → restored to 1.00 · 0.50 ≤ arrival < 0.70 → restored to arrival ⁄ 0.70 · arrival < 0.50 → bit lost
Each of the seventeen time slots owes its own coupling factor to the lapping of the quartz — ours run from 0.93 (bit 10, the weakest) to 1.00 (slot 6). At nominal gain every product clears 0.70 and the reshaper is perfect. Near the edges of the knob, those third decimal places decide which bit dies first.
03 · FAILURE
Memory as a juggling act — and two ways to drop it
STARVE · GAIN 0.99
Below unity loop gain the reshaper can no longer reach full height. The weakest crystal coupling suffers first: its pulse leaves each regeneration slightly shorter than the lap before, a geometric decay you can watch in the tube.
MEASURED — word 1337 at gain 0.99: bit 10 (coupling 0.93) falls through the 0.50 discriminator 6 laps after the knob turns, arriving at 0.478; bit 3 follows at lap 14, bit 5 at lap 19 — while bit 4's stronger crystal (0.99) clears saturation and never dies at all. The register scars each loss with an ×.
OVERDRIVE · GAIN 2.05
Too much gain and the amplifier's own noise recirculates: each lap the empty slots' residual ring is amplified above what the tank absorbs, building lap on lap — watch the faint shimmer grow in the silent slots — until it crosses the discriminator and a pulse is born that nobody wrote.
MEASURED — from nominal equilibrium at gain 2.05: slot 6 (coupling 1.00) crosses threshold on lap 12 and a phantom 1 joins the word; slot 16 ignites a lap later. The circulating value is now a number you never stored.
Both failures are computed, not scripted: the laps and bit numbers above are read out of the same recurrence that animates the tube, and the tank log timestamps each event as it happens. Store your own number and the arithmetic — not a storyboard — decides which of your bits dies first.
04 · NUMBERS
The tank, against the literature
EDSAC's long tanks were steel tubes about a metre and a half in length, run warm and temperature-controlled — the speed of sound in mercury drifts with heat, and a drifting delay is a corrupted store.
Accounts put the round trip in the half-millisecond range — ~560 microseconds by several tellings. This page's tank is slowed roughly 6,400-fold so the pulses read at human speed.
Short words of 17 binary digits is the common telling, though conventions vary by account — 18 pulse times per word with one lost to spacing is another. We circulate 17 plain digits and let the sign rest.
Everything on the dials is measured from the running simulation, not typeset: the lap counter, the mercury odometer (laps × 1.5 m at tank scale), the arrival amplitudes on the amplifier scope, and every loss or phantom in the log. Where the historical record varies, the copy hedges rather than asserts.