The relay switch is a magnetic switch used so that a large current can be switched on and off by a much smaller current. Such switches are used in car ignition switches and many electric systems which require a unit that delivers a significant amount iof current or power to be switched on or off by a low current or vltage switch. This makes the device much safer and cheaper.
When the switch above is pressed, current passes through the relay coil which produces a magnetic field. This attracts the iron bar in the right hand circuit to complete the circuit. Current can then flow though the pump. When the switch is released, current in the left hand circuit ceases to flow. The spring loaded iron bar breaks the circuit and current ceases to flow in the right hand circuit.