# Effective resistance of inductor

In a lab experiment, we connected a simple circuit: an AC voltage source, connected (in series) to a variable resistor and an inductor. We measured the current in the circuit, and the voltage that falls over the inductor. We calculated the phase difference between the voltages and used it to calculate $V_L$, and used it to calculate $R_L$, the effective resistance of the inductor. We got that $R_L(I)$ rises up to a maxima, and then decreases, but we couldn't understand why - as we understand, $R_L=2\pi fL$, so it should be constant...

What did we not understand?

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 So you're adjusting the variable resistor, and the frequency of the AC source is constant? – Art Brown Jan 3 at 2:25 Exactly. The frequency stays constant while we change the current through the resistance. – Ofir Jan 3 at 8:16

1) At very low current levels (corresponding to very low levels of magnetic field H), the inductance can be lower than nominal. (The B-H characteristic of the magnetic core material has a lower slope right at the origin.) As current increases from these low levels, the calculated inductor impedance $Z_L =2 \pi f L$ would increase and then stabilize at the nominal value.