Eddy currents in multilayer coils

This article considers an example of eddy current behaviour that has not been described previously in textbooks and physics education journals. This phenomenon involves a sharp rise in the active resistance of a multilayer coil with increasing alternating current frequency. This effect is not explained by the classic skin effect and is not related to losses in a core. The results of measurements of the frequency dependence of the active resistance of multilayer coils obtained by the resonance method and the results of calculating this dependence in the framework of the tape model of a multilayer solenoid are presented. The special case of relatively low frequencies is analysed when the classic skin effect is weak in a solitary wire. However, the active resistance of a multilayer coil is significantly higher than its DC resistance. A schematic model of the observed effect is proposed. The analysed example of eddy currents is easy to realize in an experiment and open to clear physical interpretation. It might be appropriate for students of various specialities to discuss this effect in intermediate or advanced physics laboratories.