A transformer is a electromagnetic device that steps up or steps down voltage in a electrical circuit. There are two sets of windings per leg in a transformer. Your primary side, the side you want to transform, energizes the windings in the primary coil and induces voltage in the secondary coil and your desired voltage is your output. For example if you have a 10:1 transformer, 10 being the primary and 1 being the secondary, you put 100 volts on the primary you will get 10 volts on the secondary. There is also no electrical circuit between the primary of a transformer and the secondary of the transformer. The voltage is induced into the secondary as I said before because of lines of magnetic flux being cut by the collapsing field in the a/c current passing through the primary.
