Light switches – have you ever noticed that some of them just let you switch the light on and off, whilst others give you the power to choose how bright you want the light to be? How they do this is pretty simple, so let’s look at it a bit more closely.
We’ve previously covered resistance here in electronic wires, with the contributing factors being 1) material type 2) wire length 3) cross-sectional area 4) temperature. So let’s think which of these we can change of a varying basis depending on what brightness we want.
It’s a pretty costly solution to have different wires made from different materials running all round your house just in case you want to dim the lights, and it would be hard to get a continuous scale too. Likewise cross sectional area would require a huge amount of extra wiring. That leaves us with temperature and length.
Whilst I like the idea of changing the brightness of a bulb by changing the temperature, it seems a little impractical! So that leaves us with length. You need quite a lot of wire to change the resistance so dramatically that you will notice it in the brightness of your bulb, but the dimmer switch is usually just a rotating knob. The wire is coiled up on itself so that rather than being able to choose from any point along its length, you make contact with one spot along the coil.
That way you get a lot more change in resistance with only small turns of the dial - meaning that rather than a huge distance moved you can just use a rotating switch. This type of resistor is called a variable resistor and its symbol is:
It’s used for other applications too, can you think of any?