Electromagnetic induction is when an electromotive force (emf) is induced in a conductor when lines of flux are being cut. This can occur when a magnet is moved relative to a coil of wire. This is the basis of A.C. generators and transformers. Transformers use a changing magnetic field to induce a voltage in another coil. They only work with A.C.
In a transformer, the ratio between the primary and secondary voltage is equal to the ratio between the primary and secondary number of turns, or:
This is used in the transmission of electricity: Before the electricity goes to the national grid, there is a stepup transformer, and this raises the voltage. A higher voltage results in a lower current, meaning less energy is lost due to heat, and therefore less money is wasted in the transmission of the energy.