A transformer has primary and secondary coils linked by an iron core. Alternating primary current creates a changing magnetic field that induces p.d. in the secondary. A step-up transformer has more secondary turns and a step-down transformer fewer; ordinary transformers need a changing field and do not continuously transform steady dc. Example: Explain transformer action. Ac in the primary creates a changing core field; this induces an alternating p.d. in the secondary. Consider the conditions.