RESISTANCE - FRIEND OR FOE?

Wherever electricity is flowing through a conductor there is opposition - RESISTANCE - to the movement of electrons. In the previous Fact Sheet 2 we explained how this works by using a 'helper' in the form of water flowing out of a tank. In the left water tank only little water gets through. That is because the RESISTANCE to the flow of water through the thin outlet tube is high. On the right, with its large diameter outlet pipe, there is little resistance. So lots of water will flow even if the pressure is not great.

Wherever there is an electric current flowing either through thin wires, or thick ones, the process is very similar. Thin wires have more resistance to the flow of current.

In this Fact Sheet let us examine what resistors are and how we can build one.

Generally, resistors work by converting electron flow into heat. One resistor (in disguise) is the electric radiator you all know. The wires in the radiator are thin and made from a metal that has a high resistance, anyway. The electrons have to squeeze through the rather 'narrow' passage, they push and shove. Imagine they get very angry. So they create a glow and give you the nice warmth on a cold winters day.

This effect is much stronger in the light bulb. Here the wires are thinner still. The resistance of the wire makes it so hot that it gives us bright light.

Resistors in electronic gear fulfil many useful and necessary functions. In Fact Sheet 5 we'll show how you can build an LED (Light Emitting Diode) tester. The LED needs a resistor in series with the power supply. Otherwise the LED would get damaged in a short time. In this case the resistor has the task of limiting the current to a safe value.

How do you build your own resistor?

Take a piece of paper, say 5 cm (3") square. Take the darkest and softest lead pencil you can find. The 'lead' in pencils is not lead, but really carbon. This is capable of conducting electric current. A carbon film, however, puts resistance to the current flow and that is what we want.

Now cover in crisscross fashion one side of the paper until you get a thick layer of carbon. Take a bobbin, that's a small round piece of wood or other non-conducting material, some 5 cm long. (We will talk more about conductors and insulators in a future Fact Sheet No 9) Wrap the paper, carbon layer OUTSIDE, around it. To finish you'll need two short lengths of blank wire and wrap one around each end. Make several turns to ensure the wires are wound very tight and twist them to keep them in place.

Now you can go to a friendly Radio or TV serviceman and ask him to measure how much resistance it has. He'll use his Ohm meter and tell you how many OHMS your resistor has. It only takes seconds and there should be no charge for that.

Now we have three values to play with: voltage, current and resistance. There are mathematically related by - you may have already heard of it - OHM'S LAW. To work with those figures can be fun even for those of you who normally don't like maths.

Ohm's Law will be one of the topics in our next Fact Sheet No 4.

If you do not want to play with numbers, you may go to the one that follows, which is Fact Sheet No 5.

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If you have any questions you are most welcome to send them to my mail address.

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Copyright © 1997-2010 Peter Schmedding, Child Development Projects, Canberra, Australia.