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A very accurate means of measuring current in the vicinity of zero is required to measure a certain parameter characteristic of the behaviour of an electric arc in alternating current and dynamic state. To measure this current, the authors developed a device which inserts a high value shunt (0.1 ¿¿) in series with the circuit breaker at the moment chosen for measurement. Its mechanical principle is based on the Thomson effect, which enables a very large impulse to be transmitted almost instantaneously to a moving element, by discharging a capacitor in a coil. The operation of the electronic unit controlling this system is linked with zero current and is independent of the amplitude and frequency of the former. The minimum measurable current is less than 0.1 A. This device has been produced and tested. It does not disturb the circuit. The quality and reproducibility of the measurements are excellent.