# Pencil Resistors

 Difficulty Time Required Very Short (≤ 1 day) Prerequisites None Material Availability Readily available Cost Very Low (under \$20) Safety Requires adult supervision

## Abstract

Want to know how electrical engineers "trap" the energy in a circuit to make your favorite electrical appliance? Video games, computers, phones, and many other electrical devices use "resistors" in different ways to control the electricity in a circuit. In this experiment, you can make your own resistors out of pencils, and test the effect a resistor has on a circuit.

## Objective

In this experiment you will test if the length of a pencil resistor effects the output of a circuit.

## Credits

Sara Agee, PhD, Science Buddies

### MLA Style

Science Buddies Staff. "Pencil Resistors" Science Buddies. Science Buddies, 6 Feb. 2016. Web. 23 July 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p013.shtml>

### APA Style

Science Buddies Staff. (2016, February 6). Pencil Resistors. Retrieved July 23, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p013.shtml

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Last edit date: 2016-02-06

## Introduction

The existence of electricity has been known since the ancient Greeks used to rub pieces of amber with fur to make static electricity. Benjamin Franklin is credited with the first demonstration that the electricity in lightning and static electricity are the same in his famous, but very dangerous experiment. It took hundreds of years for thinkers, inventors and scientists to learn how to control and harness the power of electricity. You can learn more about electricity in the Science Buddies Electricity, Magnetism, & Electromagnetism Tutorial.

The first great achievement was the discovery of the concept of a circuit in 1800 by an Italian named Alessandro Volta. He showed that electricity flows through a circuit, and that a circuit needs to be complete, or closed, in order to work. He also invented the first battery, and we use the word Volt to identify the units of electricity.

In 1820, Andr-Marie Ampre published his explanation of Hans Christian Orsted's discovery that magnetic needles could be deflected by an electric current. Ampre's work, later refined by James Clerk Maxwell, firmly established the connection between electricity and magnetism. The movement of electricity through a circuit is called "current", and we measure the current flowing through a circuit in Amperes (often abbreviated "amps").

The next great discovery was by a German school teacher named Georg Simon Ohm in 1826, who had been a student of Volta. He discovered that some materials slowed down, or resisted, the movement of electricity. He found out that there was a relationship between the amount of electricity in a circuit, the movement of electricity through the circuit and the resistance of the circuit. The unit for resistance, Ohms, is named in his honor.

Even though Volta, Ampre and Ohm had paved the way for the first circuits, a real use for electricity still had not been shown and it was mainly a novelty. The first useful invention using electricity was the electric telegraph in 1832, which was used to send messages by code over long distances. But the first practical invention using electricity was the incandescent light bulb by Thomas Edison in 1877.

Electricity is a very important part of our modern world and none of the modern technology we use today could exist without it. All of our modern day gadgets, appliances and electronics use the power of electricity to work. It is the careful balance of parts of a circuit, batteries, wires and resistors; and the completeness of a circuit, which allow electricity to be useful, and not harmful.

In this experiment you will put these pieces together to build your own simple circuit and use it to investigate resistors. What do resistors do, and why are they useful? How will changing the size of the resistor effect the circuit? By varying the size of the resistor, and looking at the effect on a light bulb, we will determine how resistors work in a circuit.

## Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!
• Electricity
• Circuit
• Resistor
• Current
• Conductor
• Insulator

## Bibliography

Here are some great internet resources available:

Also try these great books:

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## Materials and Equipment

The electronic parts for this project are available from Jameco Electronics.

• #2 pencils
• Ruler
• Automatic pencil sharpener
• Popsicle stick
• A coping saw (you will need an adult's help to use this)
• AA batteries (4), part #198707
• 4xAA battery holder, part #216152
• Alligator clip leads (sold in a 10-pack, but you only need 3 for the project), part #10444
• 6 V mini incandescent bulb, part #210008

## Experimental Procedure

1. Set up your circuit that you will use to test your pencil resistors, as shown in Figure 1.
1. Insert the four AA batteries into the battery holder. Make sure the "+" signs on the batteries line up with the "+" symbols inside the battery holder.
2. Connect a red alligator clip to the red wire from battery holder (in electronics, red wires are usually used for the "positive" connection).
3. Connect a black alligator clip to the black wire from the battery holder (in electronics, black wires are usually used for the "negative" connection).
4. Attach the other end of the black alligator clip to one of the light bulb leads.
5. Attach one end of a yellow alligator clip to the other light bulb lead.
1. Note: the color of this wire does not matter. Your alligator clip pack also came with green and white wires. You could use one of those instead.
2. Note: the light bulb leads are very close together. Make sure the two alligator clips do not touch, or this will create a short circuit and the bulb will not light.
6. You will connect your pencil resistors between the free ends of the red alligator clip and the yellow alligator clip.

Figure 1. The test circuit for this experiment. The twist ties are not required, but they can help keep your circuit neat by bundling up the alligator clip leads.
1. Before you start your experiment, you need to make sure your circuit works. Touch the two ends of the empty alligator clips (red and yellow) to each other, making sure to hold onto the insulated sleeve so you won't get a shock. Does your light turn on? If it does, move on to the next step. If not, go back to step number 1 and check over your circuit to see if everything is connected correctly.
2. Next you will make your pencil resistors to test in your circuit. You will be making several different resistors of different sizes by cutting pencils to different lengths and sharpening both ends of the pencil. You will need your parent's help for this part.
3. With your parent's help and using a small coping saw, cut the pencils to different lengths. The pencil lengths for this experiment should offer a nice variety of small to large sizes, and be at regular intervals, such as 2 centimeter (cm), 4 cm, 6 cm, etcetera.
4. After you cut each pencil, use the pencil sharpener to sharpen both ends of the pencil fragment. Don't worry about changing the lengths of your pencils, because you will be measuring them in the next step.
5. Use a ruler to measure each piece of pencil from tip to tip of the sharpened pencil lead. Remember to write down and keep a record of your results!
 Length of Pencil: (measured in cm) Brightness of Light: (off, low, medium, high)
1. Next, place each pencil resistor one at a time into the circuit between the alligator clips by clipping onto the pencil lead portion at the tip of each end of the pencil, as shown in Figure 2. It is important to make sure the clips are attached to the graphite and not to the wood, because wood is an insulator and is not a conductive material.

Figure 2. Connect the alligator clips to the graphite at each end of a sharpened pencil.
1. Look at the light each time you connect one of your pencil resistors to the circuit. Make a record of your observation, and try to use a number scale to describe what you see. For example, you might use a scale of 1 to 5, where 1 is dark and 5 is bright.
2. Remember that wooden popsicle stick? This is your "control" group. Put it into your circuit and rate it using the same method and scale you used to test your pencils.

## Variations

This experiment can be just the beginning to having fun building your own circuits. Here are many ways to make your experiment unique:
• Try using the same circuit set-up to test different materials around your house to see if they are insulators or conductors. You might be surprised that some common household materials can be tricky to predict!
• In our experiment, we are using a battery as a source of energy. How do you think different kinds of batteries would work in this circuit? Can you make a hypothesis of how the strength of the battery would relate to the length of pencil you could use?
• Can you think of a way to rearrange this circuit to make a battery tester? Try testing batteries around your house with your Battery Tester.
• Can you think of other energy sources to use for this experiment? Try using a solar cell, or a wind vane...
• Advanced. A light is just one way to test the amount of resistance in the circuit. Another more careful way is to use Ohm's law. First, use a digital multimeter to measure the voltage drop across the pencil. Set the multimeter to read DC volts. With your circuit connected, and the light bulb on, touch the positive probe (red) of the multimeter to the clip on the side of the pencil connected to the positive terminal of the battery. Touch the ground probe (black) of the multimeter to the clip on the side of the pencil connected to the negative terminal of the battery. Write down the voltage reading. Next, measure the current, or flow of electricity in the circuit. The multimeter should be connected in series with pencil resistor and the light bulb, and the multimeter should be set to read DC current. Write down the current reading. Now you can calculate the resistance of the pencil, in Ohms, by dividing the voltage, in volts, by the current, in amperes. This method will give you more accurate data of the effect of your pencil resistors on the voltage supplied to the light bulb.
• What if you actually want to measure the brightness of the light bulb numerically, instead of just assigning it a scale like low/medium/high? Check out the Science Buddies project Sliding Light: How to Make a Dimmer Switch

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## News Feed on This Topic

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Note: A computerized matching algorithm suggests the above articles. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more about the News Feed

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