Transformer, device for moving an alternating current by increasing or decreasing the voltage. This is an easy, inexpensive way for power companies to transport electricity over long distances. Transformers work by electromagnetic induction, where a magnetic field alters the voltage of the electric current. The current goes through a step-up transformer, which increases the voltage. High voltage electricity loses very little power when transported over long distance wires. When the current reaches its destination, step-down transformers are used to decrease the voltage to the level where it can be used by customers.