A capacitor consists of two metal plates separated by an insulator, called a dielectric; the whole assembly is able to store electricity. This store is in the form of an excess of electrons on one plate and a deficiency on the other. The type of capacitor commonly used in installation work is the electrolytic capacitor. This consists of plates of metal foil placed on either side of a waxed paper dielectric like a sandwich. As the capacitor begins to charge, its voltage increases until it is equal to the supply voltage. At the same time the charging current decreases. In an a.c. circuit, a capacitance has the effect of opposing the voltage, thus causing the circuit current to lead. Capacitors are used extensively in electrical engineering. In the field of installation work, they are mainly used for motor starting, power factor correction and radio interference suppression and discharge lighting circuits.