The Henry

Current flow in an electromagnet cannot be changed without changing the energy stored in the magnetic field. [Remember Faraday's observation. The induced voltage in a coil is equal to the rate of change of flux linkage.]

Ohm's law defines resistance for direct current as the ratio ...

In the same way the ratio Vp/Ip for alternating (sine wave) current flowing in an electromagnet is given by...

... where L is the inductance in Henrys and w (omega) is the angular frequency 2pf. Lw is the impedance of the inductor.

Note: across a pure inductance L the measured AC voltage v is 90° out of phase with AC current flow i. Impedance is properly represented as a complex number.

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