Who or what was ERNIE?

E.R.N.I.E - the Electronic Random Number Indicator Equipment

E.R.N.I.E stands for the Electronic Random Number Indicator Equipment and is a hardware random number generator created to find winners each month for the premium bond prize draw. The first ERNIE was built in 1956 by the Post Office Research Station at Dollis Hill by some of the same team of engineers who built Colossus, the world's first electronic computer. ERNIE was installed at Lytham St.Annes, a coastal town in Lancashire, England, just south of Blackpool.

ERNIE, open to view interior - image © BT Heritage

What is a Premium Bond?

A Premium Bond is a lottery bond issued by the United Kingdom government's National Savings and Investments agency. The bonds are entered in a regular prize draw and can currently potentially win up to £1,000,000 each month. The first draw using the original ERNIE was in June 1957 and had a top prize of £1,000. The total number of bond units in the initial draw was 48,487,854 giving a total of 23,142 prizes to allocate!

Premium Savings Bonds were initially issued in 23 denominations, from £1 to £500. Those for £1 have a single serial number and those with a larger value have multiple serial numbers corresponding to their face value in pounds. The example shown here for £5 has five entries with serial numbers 7EZ 854021 through to 7EZ 854025.

Each of the original bonds had a 8 or 9-digit number which comprises of a code followed by a 6-digit number. The code consists of a prefix numeral which indicates the tens of millions digit (a zero value wasn't printed giving an 8-digit number), one of 23 letters indicating the value of the bond followed by an alphabetical millions index signifying the unit millions. An example bond number of 2AS 123456 is bond number 27,123,456 with a value of A (£1).

Denomination (first letter)
Letter	Value
A	£1
B	£2
C	£3
D	£4
E	£5
F	£6
G	£7
H	£8
J	£9

Denomination (continued)
Letter	Value
K	£10
L	£20
M	£30
N	£40
P	£50
Q	£60
R	£70
S	£80
T	£90
V	£100
W	£200
X	£300
Y	£400
Z	£500

Million Value (second letter)
Letter	Value
Z	0 million
B	1 million
F	2 million
K	3 million
L	4 million
N	5 million
P	6 million
S	7 million
T	8 million
W	9 million

Design and build

ERNIE was designed by Harry Fensom, one of "the band of brothers" in a department run by Tommy Flowers, the designer of Colossus, and was built by a team lead by Sidney Broadhurst.

A test of the components required was initially built and dubbed "Little ERNIE" plus they also built a demonstration unit to show how the random number generation was calculated (this unit was rebuilt by The National Museum of Computing in 2017).

Harry Fensom, 4 February 1921 – 1 November 2010
Little ERNIE
The rebuild of the ERNIE demonstration unit by TNMOC

ERNIE in the news

How did it work?

ERNIE uses random noise generated in a neon cold-cathode tube as its primary source of random events. Two such random devices each contribute to a single digit of the nine-digit bond number so no single device failure will cause the system to fail to be random. The noise from the tube is amplified and when it peaks above a pre-set level, operates a pulse trigger. This generates pulses of a standard amplitude and width with a minimum separation between pulses giving a sequence of pulses spaced randomly over time with the bias set so that an average of 11,000 pulses per second are generated over a few minutes period. Each set of pulses feed a counter with a scale appropriate to the number or letter being generated. The first digit requires a scale of six (converted to three for the first draws until more bonds were sold), the second counter has a scale of 24, one for each of the denomination of bonds (with one unused) leaving all the other counters in a scale of 10.

Every 1/6th of a second, the noise generators are stopped so that the counters can be read without being affected by more pulses. The average of 11,000 pulses/second on a 24-scale counter means that the counter will repeat around 80 times for each required number being requested with the 10-scale counters doing many more. This is sufficiently random to allow ERNIE to stop on any of the numbers with equal possibility. The outputs of each pair of counters are combined by subtracting one number from the other meaning the digit produced will still be random even if one particular generator fails or has a cyclic component. For example, on a 10 scale digit, counter A might stop on a value of 9 while counter B stops on value 2. ERNIE combines these by moving counter B on until it passes counter A, the digit generated from the number of step required to do so, in this case, a value of 7.

This video below gives a basic instruction on how ERNIE calculates his random numbers

An excerpt from the full film, "The Importance of being E.R.N.I.E", A central office information film for the general post office.