Electromagnetism is a branch of study that combines electricity and magnetism. It is based on the principle that a changing magnetic field can induce a changing electric current or vice versa. Examples of products of electromagnetism include generators, which use motion to produce magnetism and then electricity; and electric motors, which use electricity to produce magnetism and then motion. Transformers are devices that use two coils sharing an iron core to increase or decrease voltage from a primary coil to a secondary coil.