Abstract
Large area electrodes are required to determine atmospheric charge fluxes at the earth’s surface. These also act as antennas for power line hum, swamping the small atmospheric electric signals. A battery-powered head-amplifier resolving 0.025 pA is described here, which uses active compensation to substantially cancel the interference, allowing the atmospheric charge fluxes to be resolved in the presence of the interference. The overall response is nominally −100 mV/pA, with 30 dB of 50 Hz rejection. The head amplifier can drive a low impedance line.

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