The capacitance formulae of a long cylindrical surface sensor with a gap are discussed by solving electric potential Lapalace's equation of the sensor.

通过求解带缝的长直圆柱面电容器中的电势的拉普拉斯方程，讨论该电容器的电容计算公式。

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An optical mouse uses a beam of light and an optical sensor to detect movement over a surface -- there are no moving parts.

CNN: Review: Apple Pro Mouse

However, there are problems with longer-term use, as the skin constantly produces new cells, while those at the surface die and are brushed off, meaning a new sensor would need to be attached at least every fortnight.

BBC: Health

This microscopic scale and sensitivity allows the sensor to map a high-resolution image of electrical charge that occurs over the surface of a source material (for example a human finger), when it makes contact with a thin insulating surface (ie a plastic surface).

FORBES: Can Forensic Scientists Tell How Old A Fingerprint Is?