If the electrical circuit for devices requiring a high amperage consists of a power source, a switch, and a light bulb that are directly connected, the switch and the wiring harness must be of a high capacity that can withstand the high amperage. However, through the use of a low-amperage current, a switch can turn a relay ON and OFF, which in turn, can apply the high amperage that flows to turn the light bulb ON and OFF.
The diagram on the left describes the mechanism of a relay. When the switch closes, the current flows between points 1 and 2, thus magnetizing the coil. The magnetic force of the coil attracts the moving contact between points 3 and 4. As a result, points 3 and 4 close and allow the current to flow to the light bulb. Thus, through the use of a relay, the switch and the wiring harness to the switch can be of a low capacity.