Tagged with “tnmoc”


My favourite WITCH story

Today, the National Museum of Computing (TNMoC) is celebrating the five-year anniversary of their reboot of the Harwell-Dekatron computer, also known as WITCH.

A photo of the Harwell-Dekatron under reconstruction at TNMoC in 2010. Taken from Wikimedia Commons.

The Harwell-Dekatron was originally built in Harwell in the 1950s, as part of the British nuclear program. It passed through a number of hands, before finally being decommissioned in 1973. Then it went into storage, until it was recovered by TNMoC in 2009. It moved to the museum, was restored by volunteers, rebooted in 2012, and it continues to run there today. The original news story about the reboot has more detail about the machine’s history, and how it ended up at the museum.

This computer isn’t just a static exhibit, but a working display. If you visit the museum, you’ll often see (and hear!) it running. The WITCH is powered by over 828 Dekatron tubes — a mechanical part that can hold a number from 1 to 10. It looks like a small tube, with an orange light that rotates as it cycles from 1 to 10, so you can see exactly what value it’s holding, and literally “read” the computer’s inner workings. Dekatrons also make a distinctive clackety clackety noise, and together with the visuals, the running machine is quite an experience.

A bank of dekatrons on the witch, taken by Alan Levine. From Flickr. The labels on the top row indicate the current value stored on each dekatron. Here we can read “+0998”.

The WITCH wasn’t a fast machine, even by 1950s standards. Rather than doing quick calculations, it was designed to work slowly, but run very reliably for long periods of time. Jack Howlett, Director of the Computer Laboratory at Harwell, once wrote in a report:

It took little power and could be left unattended for long periods; I think the record was over one Christmas-New Year holiday when it was all by itself, with miles of input data on punched tape to keep it happy, for at least ten days and was still ticking away when we came back.

I was once told a fun story about this Christmas run. The operators wanted to check the machine kept running, but without someone having to be in the room. So they left the phone off the hook, hanging next to the WITCH, and they’d dial in to check how it was doing. If they heard the characteristic clackety-clack, they’d know the machine was still running, and they’d rest easy. Silence, they’d know it had stopped.

I can’t remember where I first heard this story, and I have nothing to back it up. But I find the idea delightful — a machine left to run over Christmas, tracked by an analogue phone and a mechanical clack. Such an ingenious way to do remote monitoring.

Happy birthday, WITCH!