Coupling can also occur in oscillating electrical circuits (cf. Figure 1.21). An electrical version of a coupled oscillator is shown in Figure 4.14. A mutual (shared) inductor M couples together the two electrical circuits where the magnetic flux arising from the current in one circuit threads the second circuit. Any change of flux induces a voltage in both circuits. A transformer, which is used to change the amplitude of an AC voltage, depends upon mutual inductance for its operation.