IF YOU want to set an accurate standard for capacitance, the best way is to
knuckle down and count out 10 million electrons one by one, according to
physicists from the National Institute of Standards and Technology (NIST) in
Boulder, Colorado.
Any object subjected to a voltage will accumulate electric charge. The
greater the object鈥檚 capacitance, the more charge it will store at a given
voltage.
Like most other physical quantities, the unit of capacitance鈥攖he
farad鈥攊s defined by a standard. For example, the second is defined as the
duration of 9,192,631,770 vibrations produced by caesium-133 atoms. The existing
farad standard involves measuring the capacitance of an elaborate configuration
of parallel cylinders, and can take a month to give a result.
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Now, Mark Keller and colleagues at NIST want to define the farad by counting
every one of 10 million electrons placed on a pair of ultra-cold electrodes, and
measuring the voltage between them. They used a series of 鈥渢unnel junctions,鈥�
semiconductor devices that electrons can squeeze through only one at a time. The
junctions passed electrons like a tiny bucket brigade. The ratio of charge to
voltage yields a measurement in farads accurate to one part in a million because
the charge is known with absolute certainty and the voltage is measured relative
to an even more precise standard.
Keller thinks counting electrons may yield a result in a day. The current
standard 鈥渋s really a pain鈥�, Keller says.