The Harwell Dekatron weighs 2.5 tons, contains 10,000 moving parts and can work without a break for 80 hours a week.

London - With buttons, levers and flashing lights, this looks like a computer control room.

It is, however, just the computer. The Harwell Dekatron weighs 2.5 tons, contains 10,000 moving parts and can work without a break for 80 hours a week.

So what does it do? It churns out lots of sums very quickly. In other words, it’s a calculator.

When it was built in 1951, this computer was a marvel of technology. Yet only a couple of decades later, the same processing power was available in something a little smaller. Pocket-sized, to be accurate.

But for its time, the Harwell Dekatron was highly impressive.

Capable of calculating sums with great accuracy 24 hours a day, it was based at the Atomic Energy Research Establishment in Harwell, Oxfordshire, where it was used for mathematical modelling.

It was so advanced, it replaced a highly trained member of staff who had previously been number crunching on a clunking mechanical hand calculator. The Harwell used dekatrons – gas-filled counting tubes containing ten cathodes – for memory, similar to the RAM in modern computers. Paper tape was used for input and programme storage, and results were fed to a teleprinter or a paper tape punch.

It was a highly industrious addition to the staff – averaging an 80-hour week during one nine-month period from May 1952.

It was later given to a college for training purposes before it was consigned to a museum and eventually dismantled and put into storage.

The Harwell is being switched on again today for the first time in a quarter of a century at the National Museum of Computing at Bletchley Park in Buckinghamshire.

And once powered up, it will become the oldest original functioning electronic stored programme computer in the world.

The machine has been rebuilt by a team of volunteers led by museum trustee Kevin Murrell. “I first saw the Harwell Dekatron when I was a teenager in the 1970s,” he said. “It was on display in the Birmingham Museum of Science and Industry and I was very fond of it.

“When that museum closed, it disappeared from public view.

“But four years ago, quite by chance, I was looking at some photos of items in a storage centre and in the background I noticed the computer’s control panel.” The Harwell, which was by then housed at Birmingham Museum’s Collection Centre, had been dismantled into 50 pieces.

But Mr Murrell was determined. He managed to locate around 99 percent of the computer, which has 828 dekatron tubes, 480 relays, 199 lamps and three miles of wire.

He and his volunteers spent three years reconstructing the machine, using original circuit diagrams and photos.

The original designers were also tracked down for advice.

“It is terrific to see it working again as I remember it so vividly from my teenage years and it is probably what inspired me to go into a career in computing,” said Mr Murrell.

“To see it in action is to watch the inner workings of a computer, something that is impossible on the machines of today. We hope it will now enthuse another generation of children.” - Daily Mail