We have developed highly sensitive electrometers and electrostatic fieldmeters (EFMs) that make use of micromechanical variable capacitors. Modulation of the input capacitance, a technique used in macroscale instruments such as the vibrating-reed electrometer and the field-mill electrostatic voltmeter (ESV), moves the detection bandwidth away from the 1 -noise-limited regime, thus improving the signal-to-noise ratio (SNR). The variable capacitors are implemented by electrostatically driven resonators with differential actuation and sensing to reduce drive-signal feedthrough. The resonators in the electrometer utilize a balanced comb structure to implement harmonic sensing. Two fabrication methods were employed-a hybrid technology utilizing fluidically self-assembled JFETs and SOI microstructures, and an integrated process from Analog Devices combining 0.8-m CMOS and 6-m-thick polysilicon microstructures. All devices operate in ambient air at room temperature. Measured data from one electrometer with an input capacitance of 0.7 pF indicates a charge resolution of 4.5 aC rms (28 electrons) in a 0.3 Hz bandwidth. The resolution of this electrometer is unequaled by any known ambient-air-operated instrument over a wide range of source capacitances. The EFM has a resolution of 630 V/m, the best reported figure for a MEMS device.[985]Index Terms-Electrometer, electrostatic fieldmeter (EFM), electrostatic voltmeter, variable capacitor.

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A resonant wireless power transfer (WPT) system that allows for free positioning of a wearable device about a 360 o axis of rotation and simultaneous charging of a mobile phone is proposed. A 3D transmitter coil structure is introduced to provide a cylindrical charging surface with a uniform magnetic field. A prototype charging system including an amplifier and a rectifier operating at 6.78 MHz is demonstrated. The prototype delivers 1 W to a wearable device and 5 W to a mobile phone simultaneously with a power transfer efficiency of 48%.

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