Oxford Electric Bell: The battery that's outlived queens and presidents

The Oxford Electric Bell, or Clarendon Dry Pile, has been ringing for at least 179 years.

It's kept at Oxford University where it has been (now inaudibly) ringing since 1840.

It may be 19th century technology, but that's the kind of battery life your phone can only dream of.

So how does the world's most durable battery work - and how much longer can it last?