When the switch is closed the
lamp (L_{1}) in series with the inductance lights more slowly than lamp L_{2} suggesting that the
inductance resists a changing current.

A coil with an iron core has a much greater
inductance than one with an air core, and using the circuit in Figure 2 you can show that the
lamp will dim markedly or even go out when the iron core is placed in the coil. If d.c. were used,
however, the lamp would remain alight whether there was an iron core or not.

In the inductive circuit the current and voltage are not in phase; in fact, the current lags behind the voltage by 90

This can be shown mathematically as follows.

Let the current flowing through an inductance L be i= I

Now the back e.m.f. produced is E = - Ldi/dt = -wLi

The applied p.d. is therefore: -E = v = wLi

This shows that the voltage leads the current by 90

An easy way of remembering the lead and lag is to use the word