While a transformer is only one part of the circuit in which it is used, it also serves to divide any circuit in which it is placed into at least two smaller circuits, one side having energy of one voltage and the other at a voltage set by the transformer. The primary circuit is the circuit that flows into the transformer and back to the end of the circuit that feeds energy. The primary circuit supplies the power which is eventually used by the secondary circuit. Secondary circuits begin and end at the transformer, and it is this circuit which does the actual work.

In distribution networks, primary circuits are high-voltage lines. End-use customers use energy fed from secondary circuits from transformers that reduce the voltage to required levels. But voltage doesn't necessarily determine which circuit is primary. As an example, every television set has a transformer which uses a 120 volt primary circuit that feeds several hundred volts to a secondary circuit needed by the picture tube.

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