Development of techniques for measuring magnetic fields and currrents by Faraday rotation in single‐mode optical fibers has continued. We summarize the results of attempts to measure the toroidal plasma current in the ZT‐40 reversed field pinch using multiturn fiber coils. The fiber response is reproducible and in accord with theory, but the amount and distribution of the stress‐induced birefringence in this case are such that prediction of the sensor response at low currents is difficult if not impossible. The low‐current difficulty can be overcome by twisting the fiber to induce a circular birefringence bias. We report the results of auxiliary experiments with a fiber that has been twisted with 15 turns per meter and then recoated to lock the twist in place.
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