A transformer transfers power between two circuits without physical contact by using electromagnetic induction. It consists of two coils called the primary and secondary windings. Changes in the primary winding's magnetic flux induce voltage in the secondary winding according to Faraday's law of induction. There are different types of transformers classified by their application, voltage ratios, core material, and winding arrangements, such as step-up transformers which increase voltage and step-down transformers which decrease voltage from one circuit to another. Common transformer cores use either air or iron to provide a flux path between the windings.