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A new principle of operation makes feasible small, cheap potential-independent current comparators whose performance reaches or exceeds the best properties of bulky magnetic amplifiers of the second-harmonic type. Connection of a ferrite-core coil in parallel with a suitable negative resistance generates relaxation oscillations . Inherently, at large oscillations the coil ( )- characteristic is perfectly symmetric about the origin: . Provided that the negative-resistance ( )- characteristic has the same symmetry: , the oscillations are perfectly symmetrical in the sense that . Here represents either the coil flux φ, the current or the voltage . The time is and the oscillation period. External magnetomotive force shifts the coil ( )-characteristics, invalidating (1) and therefore (3). Consequently, the deviation from 50% duty-ratio of the oscillating is a sensitive and extremely stable measure of this MMF. Neither core temperature, pressure nor magnetic creep cause any zero drift. Orders of magnitude reached experimentally are: Short-time zero instability and noise: ampere-turn, time resolution: s, zero drift from -70 to +100°C: ampere-turn.