  A mechanical electroscope based on a change in the resonant frequency of a
cantilever one micron in size in the presence of charge has recently been
fabricated. We derive the decoherence rate of a charge superposition during
measurement with such a device using a master equation theory adapted from
quantum optics. We also investigate the information produced by such a
measurement, using a quantum trajectory approach. Such instruments could be
used in mesoscopic electronic systems, and future solid-state quantum
computers, so it is useful to know how they behave when used to measure quantum
superpositions of charge.
