Experiment 10

Experiment 10. Making a dimmer for your mini lamps.

STEP 1. Connect a mini bulb to a battery. In this case I chose two C cells in series, so 3 Volts.

STEP 2. I found a soft drawing pencil. Something like they use in primary grades for printing or in art materials for dark shading. I sharpened both ends and put it in the light circuit. I was careful to wrap the wire around the pencil lead and not the wood, so I had to take a knife and pare off some of the tapered wood on each end.

Compare how bright the bulb is now.  Some of the voltage is now divided between the pencil and the light so that the light is dimmer. 

Discussion:  The pencil lead is not lead at all. It is carbon. The reason we call pencil points "lead" is that originally they were made of lead. Now they are made of different densities of carbon.  Carbon is a poor conductor. It actually resists the flow of electricity, so we call it a "resistor" in the circuit.

I used my multimeter to measure how much resistance my pencil made in the circuit. The meter read 2.0 ohms.  Ohms are the measure of the amount of resistance a substance has in the circuit.

More Discussion.  You can buy resistors of almost any ohm value.  If you have ever seen inside a an electronic circuit you would have seen these devices. They are the most common devices in an electronic device.

I checked this with the multimeter. This time it read 8.3 ohms.  More resistance and less current flowing through the lamp. Thus, it was even dimmer.

STEP 4. Now that we know something about carbon as a resistance, let's see how we can use it to make a variable resistor. I took a pencil and pared off the wood from half of it. (See below).  On the end with the point I wrapped a wire and then clamped it with a clip. You could use a hair clip or a bulldog clip you use for holding papers together.

I attached another wire to a paper clip and tightly wrapped it to the end as shown below.

I used this strip of carbon in my circuit to make a dimmer for my lamp.  

Discussion:  When you turn the volume up on some kinds of electronic devices that actually have a knob, it is connected to a shaft that turns a variable resistor.  The device look like this:

Inside is a strip of carbon and a wiper blade much like my pencil and paper clip

The carbon is a strip from 1 to 3. The wiper is #2 When you want the volume low, the wiper is swept to the longest part of the carbon. When you want it loudest, the wiper moves to where there is no carbon resistance.

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