The typical magnetic amplifier consists of two physically separate but similar transformer magnetic cores, each of which has two windings: a control winding and an AC winding. Another common design uses a single core shaped like the number "8" with one control winding and two AC windings as shown in the photo above. A small DC current from a low-impedance source is fed into the control winding. The AC windings may be connected either in series or in parallel, the configurations resulting in different types of mag amps. The amount of control current fed into the control winding sets the point in the AC winding waveform at which either core will saturate. In saturation, the AC winding on the saturated core will go from a high-impedance state ("off") into a very low-impedance state ("on") – that is, the control current controls the point at which voltage the mag amp switches "on".