The answer is, of course, none. That’s if you’re lucky enough to be a fireman in Livermore, California. Livermore’s centennial light has been blazing continuously, like some electronic Olympic torch, since 1901 - perhaps making the bulb even older than the ‘lightbulb’ joke itself.

The LA Times reported this week that the centennial light has been named the world’s longest continuously burning bulb by Guinness World Records, who don’t seem too concerned that in 1976 the bulb was actually switched off - albeit for just 22 minutes.

The authorities were forced to briefly pull the plug on the bulb because it had outlived the old Fire Department HQ. It now illuminates Firestation number 6 in Livermore. Although, perhaps ‘illuminate’ is the wrong word. That’s the bulb in the photo on the right, blazing out its 4 watts of power. The far brighter strip light next to it is a more recent addition.

This bulb is a survivor. In the 1950s, long before its antiquity was realised, firefighters absentmindedly thwacked the bulb for good luck on the way out of the building. Later, in the 1970s, their sons used it as target practice for indoor basketball games. And through it all, the bulb burned on.

But not everyone is a fan. The folk of Fort Worth, Texas, thought their town was home to the world’s oldest bulb - Fort Worth Palace Bulb - before the historians took a closer look at Livermore’s. Bud Kennedy of the Fort Worth Star-Telegram seem to take the news particularly badly. Last year he took a trip to view the Livermore bulb “to kick the wall and see if I could jiggle it out of its socket”.

What is it about these two bulbs that contributes to their longevity? Some suggest it’s because they’re so rarely switched off. Environmentally-minded campaigns to switch off power for periods such as Earth Hour may have scuppered young pretenders to the lightbulb crown.

Tom Bramell, a retired firefighter in Livermore and former guardian of the centennial bulb, has his own theory. The vacuum seal around these bulbs is so perfect that air hasn’t been able to destroy the filament, he thinks. One thing’s for sure - they don’t make ‘em like that anymore.

If you have your own theories, you can always email the bulb.

Colin Barras, online technology reporter

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