Nick Hall
------------------------------- Daily Telegraph Article ----------------------
Technoturkey: the Dry Pile
By Barry Fox
IN 1840 an Oxford science student was shopping in London
and found a perpetual motion machine. As if by magic a couple of bells, on
top of some yellow pillars, were ringing merrily. The salesman assured him
that they would go on for ever.
The student bought the machine and was mocked by his
colleagues. But he had the last laugh. The machine is still going strong
and likely to carry on chiming for several hundred years more. The bells
are likely to wear out before the mysterious source of power dries up.
Visitors now come from all round the world to see the Dry Pile, now inside
a bell jar in the Clarendon Laboratories.
Each pillar is a stack of 2,000 zinc foil and paper disc
pairs, impregnated with manganese dioxide. The yellow colour comes from a
coating of molten sulphur which keeps the internal moisture content
stable. There is just enough water trapped inside to serve as an
electrolyte, but not enough to cause internal short circuits.
Each pillar generates around 2,000 volts, but at a
current of only 1 nanoampere. During the war, the Admiralty needed a very
high voltage, very low current source for the image converter tube of a top
secret infrared telescope. An Oxford physicist working there remembered the
Dry Pile. The Navy built a large number of replica Dry Piles which
successfully delivered the 3,000 volts needed for the lead sulphide cathode
of the tube.
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"That which a man cannot afford to loose owns him"
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