E.R.N.I.E.

Electronic Random Number Indicator Equipment
Mark I · 1957–1973

ERNIE chose Premium Bond winners from genuine physical randomness, the unpredictable electrical noise inside neon tubes, rather than from a mathematical formula. What follows is a working simulation of how it did that.

Thermal Noise Generators
000000
Pulses per second: ~11,000

Where the randomness begins. The neon tubes emit electrical pulses in no predictable pattern. The machine does nothing clever with them. It counts.

Ring Counters
D1: 0 D2: 0 D3: 0 D4: 0
D5: 0 D6: 0 D7: 0
L1: A L2: A
7 digit counters + 2 letter counters

These counters sample the pulse stream. Whatever value a counter holds at the instant it is read becomes one character of the bond number: seven digits and two letters.

Generated bond number
READY TO GENERATE

Teletype Output

ERNIE Mark I - Electronic Random Number Indicator Equipment Post Office Research Station, Dollis Hill - 1956 Commissioned for Premium Bonds Prize Draw Status: READY Waiting for operator command...
0
Numbers Generated
~2000
Numbers/Hour
0
Operating Time (min)
1957–73
Service Period

About ERNIE Mark I

Designer: Tommy Flowers (Colossus designer) & Harry Fensom

Location: Lytham St. Annes, Lancashire

Size: Approximately the size of a van

Technology: Thermal noise from neon cold-cathode diodes

ERNIE Mark I was the world's first purpose-built electronic random number generator for selecting Premium Bond winners. Built by the same team that created Colossus at Bletchley Park, it used pairs of neon diodes generating thermal noise to create truly random numbers.

The machine operated by counting electron pulses through neon tubes at approximately 11,000 pulses per second. Every 1/6th of a second, the counters were read to generate random digits and letters. This revolutionary approach ensured genuine randomness, unlike mathematical pseudo-random algorithms.

During its 16-year service life, ERNIE-1 generated millions of winning Premium Bond numbers, creating thousands of winners and transforming the British savings landscape forever.