Operating an electrical switch is like turning on a water faucet. Behind the faucet (or switch) there is a source of water (or electricity), a way to transport it, and pressure to make it flow. The faucet's water source is a reservoir or pumping station. A pump provides enough pressure for the water to travel through the pipes. The switch's electrical source is a power generating station – a dam, or a coal or natural gas power plant. A generator provides the pressure for the electrical current to travel or flow through electrical conductors or wires. Three factors determine the resistance of a substance to the flow of electricity: • What it is made of. • Its size. • Its temperature. Substances with very little resistance to the flow of electrical current are called conductors. Examples are metals.