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)
Denomination (continued)
Million Value (second letter)
Z0 million
B1 million
F2 million
K3 million
L4 million
N5 million
P6 million
S7 million
T8 million
W9 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

British Movietone news - PREMIUM BONDS.
Dr Charles Hill, the Postmaster General(1955-1957) shows the original demonstration unit, a rebuild of which you can see at The National Museum of Computing.

British Movietone news - Premium Bond No.1. The Lord Mayor buys the very first Premium Bond.

British Movietone news - The First Winners.
Mr Ernest Marples, Postmaster General(1957-1959) starts the very first draw.

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.

Printing the results

A valid bond number from the random number generators is then printed onto a number of hard copy teleprinters. Each bond number is printed with a generated incremental number against it as the largest cash prizes are given to the first winners working down to the smallest, so it is important to keep the numbers in the correct order.

On the console desktop are mounted two Post Office No. 7B page printers (made by Creed), these are set flush with the top surface and accessible via locked drawers at the sides of the console. These are the main teleprinters and will print a duplicate of all the random numbers generated. The left-hand printer was used to print the full untouched copy of the full draw while the right hand printer output was cut up into sections of 100 numbers each (ERNIE automatically printed a few spaces every 100 numbers generated). This subset of 100 was then chopped further into sections of 25 numbers which was taken to the draw control so that they could manually confirm each of the winning numbers was correct and in the right order.

One of the main No. 7B printers - image © BT Heritage

A set of eleven pairs of teleprinters were also installed in the file room, these were known as the file printers or sometimes spur printers (or SPU for short). The files were a manual card storage system which held photocopies of all registered bond numbers. Each set of files were split into denominations or groups of denominations to make it easier to search (for example, all the A (£1) denominations together and B,C & D (£2,3 & 4) all together, the latter being less popular sales than the A). Each pair of teleprinters attached to a file group consisted of a single GPO No. 7B page teleprinter and a GPO No. 11A teleprinter which prints on 3/8" wide gummed paper tape. (Creed referenced the 11A teleprinter as a model 47).

A section of the manual files - image © BT Heritage

Spur/File printers for denomination A - image © BT Heritage

Once a number was generated by ERNIE and designated as ready to print, the first two values of the bond number were read, and this switched the output to the correct pair of teleprinters dependant on the 10 million digit and denomination. The K (£10) and Z (£500) denominations were split out into two separate sections in the files (10 million and any others) as they were very popular bond denominations. Each set of file printers also included a switch to alert the main console operator should a problem occur (for example a paper jam) which allowed ERNIE to be paused while the problem was resolved.

Where is ERNIE now?

ERNIE ran successfully for 15 years before being retired and replaced by a newer model, ERNIE 2, in 1973. This was mainly to speed up the production of numbers as premium bonds became more popular.

Science Museum Group. GPO ERNIE I. 1990-298. Science Museum Group Collection Online. https://collection.sciencemuseum.org.uk/objects/co62675

The original ERNIE 1 is now in the collection of the Science Museum in London and was on display between 2008 and 2015. High resolution images are available here : Science Museum Collection.

It is noted that the version of ERNIE that the Science Museum have is missing the original console and is not mounted in the original racks. Further, the redundancy table is completely rearranged from the original specifications. It may be that this is the original ERNIE which was upgraded or in the process of being upgraded and had it's components mounted on a stand-alone plinth or it is possible that this may be an exact spare duplicate machine which was at Dollis Hill for testing and upgrading of new functionality.

ERNIE's legacy

ERNIE has been replaced with a newer, faster version roughly every 15 to 16 years since the original as more bond numbers were required in a shorter time. ERNIE 5, the current machine released 1st March 2019, is powered by quantum technology, which uses light. This new technology allows ERNIE to produce enough random numbers for a monthly prize draw in just 12 minutes – 42.5 times faster than its thermal predecessor at the end of its number-generating career.