What is the purpose of the transformer in a linear power supply?

A transformer is an electrical component made of two or more coils of wire wrapped around a core. Its primary function in a linear power supply is to step the 120-volt AC mains voltage down to a lower, safer AC voltage (such as 15 to 18 volts) that the rest of the supply can use. Because the primary and secondary coils of the transformer are linked only by a magnetic field and not by a direct electrical connection, the transformer also provides critical galvanic isolation from the dangerous AC mains. It is important to remember the sequence of a power supply: the transformer only changes AC voltage levels. It does not convert AC to DC (the rectifier does that), nor does it smooth the current (the filter does that).