Types. Open-Circuit:—Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding.

Fig. 101. Section of Open-Circuit Impedance Coil
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Fig. 102. Open-Circuit Impedance Coil
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Fig. 103. Closed-Circuit Impedance Coil
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