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This non-intrusive power monitoring apparatus provides circuit information without coming in contact with the circuit. The apparatus comprises a capacitive pickup, circuitry for sensing a signal from the pickup, an Analog to Digital Converter, and a digital filter that rejects low-frequency disturbances. The sensor’s design allows an untrained user to place the device in a suitable position to measure a physical parameter (electric or magnetic field) produced by existing installed equipment. In order to maximize the dependence of gain on plate-to-wire distance, the system takes a differential measurement of two vertically stacked sensor plates. Doing so neatly selects the signal from a nearby wire and rejects signals from distant wires. The circuitry of the sensor is optimally designed to be fast enough to respond to the fluctuations in monitored power and strong enough to obtain usable voltage signals out of the amplifier.