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It may be caused by ohmic resistance or self-induction.

It should be remembered that an impedance coil obstructs the passage of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force.

This is greater than its ordinary ohmic resistance due to the skin effect.

It is impossible to overcome it entirely, but merely in part, for the ohmic resistance cannot be overcome.

The coils 2 and 3 are made to have high impedance, while 1 and 4 are so wound as to be non-inductive and, therefore, offer no impedance save that of their ohmic resistance.