Loop probes are used to measure the magnetic field nearby the rails of the rail-gun, and the total current can be measured by these loops too. Several loops are placed at one side of the rails along a straight line, and the transient current can be calculated with the measured magnetic field waveforms and distances between probes and currents for the long line current models. According to the error of fixing probes, and the position shift of the current for skin and adjacent effects and electromagnetic forces, distances between currents and probes are hard to get for each shot. If two probes are used together, it is easier to measure the single line current waveform. It is needless to get the distance between probes and currents beforehand. The current waveforms can be calculated only with the calibrated distance between two probes and field waveforms. For the rail-gun's rail currents, which have the same amplitudes but reverse directions, we can use three loop probes. The current waveforms can be easily got from the three magnetic field waveforms without knowing the shift positions of the rail currents. The commercial PEARSON current monitor model 1423 is used to calibrate and verify the results. The results are compared with each other for the single loop, two loops and three loops. The results show that it is proper to measure the transient currents with one loop or two loops, while it would be better when the three loops are used. The method is useful especially for the situations that the huge ROGOWSKI current probes are hard to fix with limited volume space.
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