Primary Coils: The primary coil is defined as that coil whose current is driven by an outside alternating-current source.
The primary converts the energy in the input current into a varying magnetic field. This varying magnetic field induces a voltage in the secondary coil(s), thus transferring electrical energy without having electrical contact.
Secondary Coils: The secondary coils are defined as those coils whose voltage is induced by the varying magnetic field produced by the primary coil.
Most power and signal transformers have one secondary coil which is used as the power output of the transformer. An LVDT has two secondary coils whose outputs are balanced (nulled) against one another.