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Project Summary

Difficulty	6
Time required	Short (several days)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	Since this experiment deals with electricity, it is best to have an adult supervise. A mild to moderate electric shock is possible if care is not taken. Follow all directions as stated.

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Objective

Learn about one of the building blocks for all types of circuits—the capacitor. In this science project you will build a Leyden jar capacitor out of common household materials and determine how much charge is stored as you test different charge cycles.

Introduction

Static charge is the buildup of charge on an object. In 1745, Ewald Georg von Kleist invented a tool to capture and store static charge. At approximately the same time, Pieter van Musschenbroek from Leiden, the Netherlands, invented a similar tool. This tool is called the Leyden jar. The Leyden jar provided early electrical experimenters with a way to store electrical charge and then move it to another place to use.

Leyden jars come in all shapes and sizes, but they all have the same design. The jar, which serves as the insulator, separates an inner conductive layer (or electrode) and an outer conductive layer (also an electrode). Examples of a conductive layer are aluminum foil and gold leaf. There is usually a metal rod with a ball on top placed in the mouth of the jar. The rod, or a chain connected to the rod, touches the inner conductor and is the conduit for electrical charge to the jar.

Figure 1. Inner workings of the Leyden jar. (HowStuffWorks.com, 2007.)

Benjamin Franklin was an early electricity experimenter. He built and experimented with the Leyden jar. One of Franklin's favorite experiments was the "circle shock." In the circle shock, a group of people hold hands and one person at the end of the human chain holds the outside of the Leyden jar, while the person on the other end of the human chain touches the inner conductor. If there is enough charge in the jar, every person in the circle will feel a shock. Don't try this experiment, because you can get a nasty shock!

The original Leyden jar held water as the inner conductor. Early electrical experimenters thought that the charge was held in the water. However, Benjamin Franklin was the first person to figure out that the charge in the Leyden jar is located where the insulator meets the electrode (in this case, the water). Thus, water is not required and can be replaced by attaching an electrode, such as aluminum foil, to the inside of the jar. Franklin also connected several Leyden jars together and created what he called a battery. This is not a battery like we know today, but it was a way to store lots of charge for Franklin's electrical experiments.

The Leyden jar is an interesting device because it is the precursor to the modern capacitor. A capacitor is an electrical component that is used to store charge, and along with the resistor and batteries (or voltage supply), forms the basis of all circuits. Both capacitors and batteries are similar in that they are components that store charge. The major difference between a battery and a capacitor is that a battery produces charge through a chemical reaction. A capacitor is much simpler as it can't produce charge, it can only store charge. Capacitors are used in timer circuits and logic circuits and a variety of applications. For example, the screen on the Apple® iPhone^TM is a capacitive touch screen.

In this project, become Ben Franklin and build your own Leyden jar. You will also build an electrophorus. An electrophorus is a tool that is used for transferring charge to the Leyden jar. Try using small jars that you can find around the house. Experiment with how much charge your Leyden jar can hold. Remember that even a small jar can store a lot of charge, so be very careful or you can get a painful shock.

Figure 2. Leyden jar battery. (Bakken Library and Museum, n.d.)

Terms, Concepts and Questions to Start Background Research

• Static charge
• Leyden jar
• Benjamin Franklin
• Battery
• Capacitor
• Electrophorus
• Ground
• Static electricity generator
• Charge dissipation

Questions

• What kinds of materials can you use to create static charge?
• What kinds of jars can you use to store charge?
• Does changing the size of the electrodes affect the amount of stored charge?
• Is there a limit to the amount of charge the Leyden jar can hold?
• How long will the Leyden jar hold the charge? An hour? A day? This is called charge dissipation and this is a key factor that circuit designers evaluate.

Bibliography

• Check out the "Charge and Carry" project on the Exploratorium website:
  Exploratorium. (n.d.). Charge and Carry: Store up an electric charge, then make sparks. Retrieved March 25, 2008 from http://www.exploratorium.edu/snacks/charge_carry.html
• Also take a look at The Bakken Library and Museum's website. The Bakken has lots of interesting information about early electrical experiments and equipment:
  The Bakken Library and Museum. (2007, April 6). Leyden jars, condensers, etc. Retrieved March 25, 2008 from http://www.thebakken.org/artifacts/Leyden-jar.htm
• The following website shows different homemade Leyden jars and has some additional sources:
  Leyden Jars. (n.d.). Retrieved March 25, 2008 from http://www.alaska.net/~natnkell/leyden.htm
• This website has a good general page on capacitors:
  Brain, M. and Bryant, C. (2008). History of the Capacitor. Retrieved March 25, 2008 from the howstuffworks website: http://electronics.howstuffworks.com/capacitor3.htm

Materials and Equipment

• Electrophorous Assembly
  • Disposable aluminum pie pan
  • Styrofoam^TM cup
  • Tape (regular Scotch® tape will work fine)
• Leyden Jar Assembly
  • Small plastic jar with lid; a film canister, which is generally made with HDPE (high-density polyethylene) plastic, works very well
  • Hammer
  • Nail (a plain metallic nail that is slightly longer than the jar you are using)
  • Small piece of aluminum foil
  • Spoon
  • Tap water
  • Paper towel
• Charging the Electrophrous
  • Acrylic sheet, 11"x14" (available at hardware stores and should be larger than the pie pan)
  • Wool cloth (sweater, socks, or wool felt)
  • Assembled electrophorous
  • A stable table
• Measuring the Charge
  • Small piece of an aluminum sheet (an aluminum cover for an electrical box works well, as in Figure 6 below); available at hardware stores
  • Large, flat piece of Styrofoam
  • Plastic travel soap dish (clamshell variety); the soap dish should be rectangular
  • Scissors
  • A piece of insulated wire, 6" long
  • Wire strippers
  • Tape (regular Scotch tape will work fine)
  • Metric ruler
• General
  • Fine-tip pen
  • Insulated tongs
  • Lab notebook
  • Graph paper

Experimental Procedure

Safety Note: Adult supervision is recommended for this project. While the amount of current that can flow from this device is low, the amount of charge held in the device can cause a mild to moderate shock. To safely discharge the Leyden jar, take insulated tongs and touch the outside foil electrode to the nail, while holding onto the insulated handles. Always exercise caution when dealing with electricity and electrical parts.

The first step in this experiment is to make the Leyden jar and the electrophorus that is used to charge the Leyden jar.

Electrophorus Assembly

1. Take the Styrofoam cup and place the wide end of the cup in the middle of the inside of the aluminum pie pan. Tape the cup to the pie pan. Just tape the edge of the cup. You don't need too much tape. See Figure 3.

   Figure 3. Simple electrophorus.

Leyden Jar Assembly

1. Gently hammer the nail through the top and middle of the plastic jar's lid. Make sure that the nail almost hits the bottom of the jar when the lid is back on and that it is straight and not tilted in one direction or another. Ask an adult for help. Then take the lid off and set it aside.
2. Take the piece of aluminum foil and wrap it tightly around the outside of the jar. Tape the edge of the foil to the jar. Rub the aluminum foil with the back of a spoon to make sure that there are no gaps between the foil and the jar. Make sure that the edge of the foil is not too close to the top of the Leyden jar. If it is too close, there could be a spark-over and no charge would be held in the Leyden jar.
3. Fill the jar with tap water, about 80% full. Make sure that the outside remains dry. Dry off any moisture with a paper towel.
4. Snap the lid back onto the jar. Wipe any excess moisture off the outside of the jar. See Figure 4 for a completed Leyden jar. Note that the nail is insulated from the outer electrode.

   Figure 4. Leyden jar made from a film canister.

Charging the Electrophorus

1. Place the acrylic sheet on a table. If the piece of acrylic is new, rub it with the wool for about 1 minute until you create a static charge. Pick up the acrylic sheet to discharge it. Now you are ready to start the experiment. For the experiment, place the acrylic sheet on the table and rub the acrylic for 15-30 seconds to create a static charge. You might even hear the static charge popping and crackling while you are rubbing, signaling that it's ready to use. By rubbing the acrylic with the wool you are creating an excess of electrons on the acrylic.

   Note: Be consistent in rubbing the acrylic sheet. Be sure to rub for the same amount of time for each trial. For example, if you rub the acrylic sheet for 30 seconds and then transfer the charge, you should keep rubbing the acrylic sheet for 30 seconds on each successive charge cycle. Charge cycles are explained below.

   Figure 5. Rubbing the acrylic sheet with a piece of wool.

2. Place the electrophorus on top of the acrylic sheet, touching only the Styrofoam cup. You may hear a little bit of crackling and popping. The electrons on the acrylic sheet repel the electrons in the electrophorus. Touch the edge of the pie plate for just a second. You may get a little zap. When you touch the electrophorus, you create a path for the repelled electrons to go. Now the electrophorus is positively charged.

Charging the Leyden Jar

1. Now it is time to transfer the positive charge to the Leyden jar. Pick the Leyden jar up, only touching the outside foil. You will be acting as a ground.
2. Pick the electrophorus up by the Styrofoam cup. Don't touch any other part of the electrophorus.
3. Touch the head of the nail to the positively charged electrophorus (just the pie plate part). You may hear a zap when the electrophorus gets close to the head of the nail. You may also see a small spark. For the purpose of this project, Charging the Electrophorus through step 3 of Charging the Leyden Jar is called a charge cycle.
4. Once you've started the experiment and you proceed with your trials, be sure to note in your lab notebook how many times you charged the Leyden jar and the duration of the time that you rubbed the acrylic sheet.

Measuring the Charge

The next step is to determine how much charge is stored in the Leyden jar. Assemble your measurement setup.

1. Wedge the piece of aluminum sheet upright into the Styrofoam sheet. The aluminum piece should be perpendicular to the Styrofoam sheet. See Figure 6.
2. Snip off the lid of the clamshell soap dish with scissors. If necessary, use the scissors to carve a little notch in the short end of the soap dish in order to accommodate the nail. Ask an adult for assistance.
3. Strip a little bit of the insulator off both ends of the insulated wire, using wire strippers. Tape one end of the wire to the aluminum sheet. Tape the other end of the wire to the outside of the Leyden jar, on the foil. Make sure that the wire is in good contact with both the aluminum sheet and the foil of the Leyden jar. However, you don't want to over-tape the end of the wire to the foil because you will be taking the end off and putting it back on several times. Be careful not to rip the foil.
4. Place the Leyden jar on its side and place it into the soap dish. The Leyden jar should lie along the longer side and the nail should sit in the notch. The top of the nail head should be flush with the edge of the soap dish. Tape the Leyden jar to the edges of the soap dish so that it doesn't move around. As mentioned above, do not over-tape the Leyden jar because you need to keep the foil in one piece. Put the soap dish/Leyden jar about 3 inches in front of the aluminum plate. Place a ruler alongside the soap dish, with one end as close as possible to the aluminum sheet. Now there is a frame of reference for measurement.

   Figure 6. Testing assembly.

5. Before you begin your experiment, create a data table in your lab notebook, like the one below:

   Leyden Jar #1 (HDPE film canister)	Rubbing Duration	Spark Length After 1 Charge Cycle	Voltage After 1 Charge Cycle	Spark Length After 5 Charge Cycles	Voltage After 5 Charge Cycles	Spark Length After 10 Charge Cycles	Voltage After 10 Charge Cycles
   Trial #1
   Trial #2
   Trial #3
   Average

6. Now you're ready to start your experiment. Remember, you have already completed one charge cycle for Trial #1. Start to move the soap dish slowly toward the aluminum sheet, making sure that you don't touch the aluminum sheet or the Leyden jar. Just touch the soap dish. At some point, you will get close enough that the Leyden jar will discharge and a spark will be created. When the Leyden jar discharges, you will see and hear the spark. Stop moving the soap dish when you see and hear the spark.
7. With the fine-tip pen and the ruler, mark the Styrofoam with where you stopped the soap dish, using the edge of the soap dish closest to the aluminum sheet as your point of reference. Measure the distance between the aluminum sheet and the leading edge of the soap dish. This distance is the length of the spark. Note this data in your data table.
8. Repeat Charging the Electrophorus through step 3 of Charging the Leyden Jar and the Measuring the Charge process two more times for Trial numbers 2 and 3. Measure the length of the spark each time and note it down in your lab notebook.
9. Then repeat the experiment three times for 5 charge cycles, and three times for 10 charge cycles. Remember, a charge cycle is Charging the Electrophorus through step 3 of Charging the Leyden Jar.
   • After three trials with one charge cycle, charge the electrophorous 5 times in a row, transferring the charge to the Leyden jar each time.
   • Proceed with Measuring the Charge.
   • Repeat this for three trials.
   • After three trials with 5 charge cycles, charge the electrophorous 10 times in a row, transferring the charge to the Leyden jar each time.
   • Proceed with Measuring the Charge.
   • Repeat this for three trials as well.
10. Remember to note down the length of the spark each time.

Determining Voltage

1. When you have the distance marked down, you can determine the voltage of the discharge or spark that you saw. It takes voltages of 30,000 volts (V) to make a spark 1 centimeter (cm) long in air with a pressure of 1 atmosphere (atm), or 30,000 V/cm @ 1 atm. This means that if you see or hear a spark at 1 millimeter (mm), which is 0.1 cm, then that spark had a voltage of 3000 V.
2. Air is an insulator. However, when the nail gets sufficiently close to the aluminum sheet, the voltage between the two is high enough to strip electrons from the air molecules. The air is then ionized. Ionized air allows current to flow between the nail and the aluminum plate. Knowing the voltage produced in the spark, can determine the amount of charge that the Leyden jar can hold.
3. Plot the data in a graph where the number of charge cycles is on the x-axis and the voltage in the corresponding spark is on the y-axis. Does the voltage increase with the number of charge cycles or does it stop changing after a certain number of charge cycles?

Variations

• Try using jars made of different materials. For example, try using a small plastic food container or a small wide-mouthed glass jar. Capacitance depends upon the material that the jar is made of and the size of the electrodes, so try finding jars of similar volume. Follow the design shown in Figure 1 and make sure that the inner electrode is isolated from the outer electrode.
• Try omitting water as the inner electrode and try using foil as the inner electrode. Is there a difference in charge storage?
• How long will the Leyden jar hold the charge? An hour? A day? This is called charge dissipation and this is a key factor that circuit designers evaluate. Charge dissipation occurs when charge leaks out of the Leyden jar into the air. Charge the Leyden jar and then let it sit for 1 hour. Test the voltage in the spark using the test assembly above. Try charging the Leyden jar with the same number of charge cycles and then letting it sit for 5 hours. Test the voltage in the spark using the test assembly. Is there a difference? How long will the Leyden jar hold the charge?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

Michelle Maranowski, PhD, Science Buddies

This project is based on the "Charge and Carry" project on the Exploratorium website:
Exploratorium. (n.d.). Charge and Carry: Store up an electric charge, then make sparks. Retrieved March 25, 2008 from http://www.exploratorium.edu/snacks/charge_carry.html
The author would like to thank Eric Muller of the Exploratorium's Teacher Institute for helpful discussions.

Apple® is a registered trademark of Apple Inc.

iPhone^TM is a trademark of Apple Inc.

Styrofoam^TM is a registered trademark of The Dow Chemical Company.

Scotch® is a registered trademark of 3M.

Last edit date: 2008-04-25 12:00:00

Career Focus

If you like this project, you might want to think about career opportunities in Electricity & Electronics.

Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines. Learn more about this career: Electrician.