A triple-range portable electrometer suitable for the precision measurement of currents of the order 10−11 to 10−14 amp. is described. Current is measured in terms of the automatically registered time interval required to charge a condenser up to a certain voltage. The use of negative feedback and special operating conditions for the input FP-54 Pliotron allow current to be simply determined to within 0.1 percent. Radio-type dry batteries, which operate the instrument completely except for a timer, are contained within the amplifier and will last at least 250 hours.
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N. T. Seaton (1949) studied this question.
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