Using high voltage equipment in the classroom can be exciting and educational for students, but most students AND teachers do not realize the real-life applications of such devices in their everyday lives. Although high voltage components are used in many of today’s electronic gadgets, being able to demonstrate the principles of this electrical phenomenon can be confusing for students and may present a safety issue for the teacher if not properly used. A Leyden Jar, Wimshurst Machine and Tesla Coil demonstrate the basic fundamentals of electricity while showcasing these historically-rich devices that most students have never seen. At the most basic level, the electrical forces between electrons and protons drive the underlying concepts behind these devices, whether it is the storage of electrical charge in a Leyden Jar or the Electrical Potential Energy produced by a Tesla Coil. All of these electrical phenomena are governed by the same three basic principles;
1. Electrons are the “bits” of the atom that can be removed or added.
2. Electrons are repelled by other electrons and attracted by the protons. Or, in a simpler way… Two negatively-charged particles repel and two positively-charged particles repel while a positively-charged and negatively-charged particle attract.
3. Electrons tend to move from a greater negative concentration to a “place” where there is a lower negative concentration.
The following collection of demonstrations utilizing a Leyden Jar, Wimshurst Machine and a Tesla Coil, are designed to “spark” student interest in learning about basic concepts of electricity.
Part 1 – The Leyden Jar
Ever heard of a Leyden Jar? It’s been around for over 200 years and is the forerunner of the modern day capacitor. The Leyden Jar is a device that “stores” static electricity between two electrodes on the inside and outside of a container. The gentleman who invented it tested it on himself and stated that “my whole body was shaken as though by a thunderbolt.” It was invented by Pieter van Musschenbroek in 1745 in Leiden, Netherlands, which gave the invention its name. It is essentially an early form of a capacitor. In the early years of electricity research, scientists had to resort to large insulated conductors to store charge so the Leyden Jar provided a much more compact alternative and they were able to store substantial charge receiving a significant shock from the device when discharged. A large Leyden Jar was once discharged through seven hundred monks who were holding hands. They flew up into the air simultaneously!
The original form of the device was just a glass bottle partially filled with water, with a metal wire passing through a cork closing it. Scientists believed that a form of liquid would be the most suitable for storage of charge. A few years later, however, researchers had learned that water was not necessary, but a metal hull inside and outside the jar was sufficient for storing electrostatic energy. Lining the interior and exterior of the glass jar with conducting metal foil proved to be the most efficient and effective way of storing charge. The foil is lined to the mouth of the jar, preventing the charge from arcing between the foils. A rod electrode projects through the mouth of the jar, electrically connected by some means (usually a chain) to the inner foil, to allow it to be charged. The jar can be charged by any source of electric charge, connected to the inner electrode while the outer foil is grounded. The inner and outer surfaces of the jar store equal but opposite charges. When the brass rod is connected to a source of electricity, current travels through the rod and charges the inner foil. Current cannot pass through the glass, but the foil on the outside becomes charged by induction if it is properly grounded. The outer foil has a charge opposite to the charge inside the jar. When the flow of current into the jar stops, a charge remains stored in the jar. If the inner layers of foil and outer layers of foil are then connected by a conductor, their opposite charges will cause a spark that discharges the jar. Early experimenters found that the thinner the dielectric (insulated cup), the closer the plates, and the greater the surface, the greater the amount of charge that could be stored at a given voltage.
Experimenting with large Leyden Jars improperly can be very dangerous. In 1783, while trying to charge a battery during a thunderstorm, Prof. Richmann was killed by accidentally getting too close to a jar with his head. He is the first known victim of high voltage experiments in the history of physics. Benjamin Franklin was lucky not to win this honor when he performed his kite experiments. Franklin had later investigated the Leyden Jar and concluded that the charge was stored in the glass, not in the water, as others had assumed.
Using your Dissectible Leyden Jar
1. The dissectible Leyden Jar can be charged by a student using a plastic rod and animal fur (…a PVC pipe or plastic golf tube works quite well) rubbed together and touching the top electrode several times. After charging, the jar can be discharged with a conductor on an insulated handle by shorting the inner and outer can to show charge storage.
2. Charging the Leyden Jar again, the teacher is able to lift out the inner can with an insulated tool or with care, by hand. At this point the parts of the jar are safe to handle, and the plastic cup jar can be lifted out, the inner and outer cans touched to each other, or touched to the glass jar in any combination. You can even give the pieces to the students to handle. When the Leyden jar is reassembled, the last step of inserting the inner can must be done carefully. The students will find that the jar is still charged and can be checked by shorting its terminals and drawing a spark! The amount of capacitance (The ability of a body to hold an electrical charge) for a Leyden Jar (Capacitor) was originally based on the measured number of ‘jars’ you had lined up of a given size, assuming reasonably standard thickness and composition of the glass. A typical Leyden Jar of one pint size has a capacitance of about 1 nF (NanoFarad).
3. You can make your own Leyden Jar!
a. Using an empty film canister, place the lid on the film canister and push a nail (…a large dissecting pin works well!) down through the center of the lid.
b. Wrap the bottom 2/3 of the file can with aluminum foil. Taping it won’t hurt anything.
c. Fill the film can with water. Make sure that it’s full enough so that the nail touches the water. Now you can charge it…
d. Using a plastic rod and animal fur, rub the plastic rod with the animal fur and hold the Leyden Jar by the aluminum foil.
e. Charge the jar by touching the charged plastic rod to the nail stuck through the lid of the Leyden Jar and add more charge to the Leyden Jar over and over again.
f. Now discharge the jar by touching the aluminum foil with one finger AND the protruding nail with the other finger and you will “feel” a jolt!
The Wimshurst Machine is an electrostatic device for generating high voltages developed around 1880 by British inventor James Wimshurst. It consists of two large oppositely-rotating discs mounted vertically, two cross bars with metallic brushes, two Leyden Jars for charge storage and a spark gap formed by two metal spheres. This machine separates electric charges through electrostatic induction. In a Wimshurst Machine, the two insulated disks are turned mechanically. Their metal plates rotate in opposite directions with two pairs of copper brushes removing charges from the plates. These are collected by metal combs with points placed near the surfaces of each disk and causing an imbalance of charges to be stored and amplified in the two Leyden Jars. When the charge difference becomes too great, a spark will jump across the gap, depending on the gap distance, plate sizes, Leyden Jar sizes and the humidity of the room. A typical Wimshurst Machine can produce sparks that are about a third of the disk’s diameter in length and several tens of microamperes. The Wimshurst Machine from Arbor Scientific is rated at 7µA. The maximum Electrical Potential Difference is approximately 75,000V.
Using Your Wimshurst Machine
The Wimshurst Machine works best on a dry day, with the disks very clean and turning the crank rapidly charging the disks and Leyden Jars to a very high voltage. In humid weather, a hair dryer can be used to dry the machine and make it work.
a. Basic spark discharge. Separate the ball terminals by a small distance. Turn the crank to create opposite charges in the terminals. When the voltage difference between the terminals is sufficient, a spark will form. (The maximum spark distance will vary depending on the humidity.
b. Effect of Leyden Jars. Using the thumbscrews, remove the metal bar at the base of the apparatus to disconnect the Leyden Jars. Observe the change in the size and frequency of the sparks when the charge is delivered only to the terminals.
c. Hold a small circuit board by the insulated area and connect the Wimshurst to the circuit. By cranking the machine, a complete circuit can be shown “flashing” at each discharge of the machine.
d. Using a plastic protractor or some other thick piece of plastic, hold the plastic between the terminals and the “spark” will jump around the plastic (insulator). Do it in the dark for maximum viewing!
e. Volta’s Hail Storm. Separate the ball terminals by a large distance. Connect one to the top terminal of the Volta’s Hail Storm apparatus (P6-3320), and the other to the bottom. Turn the crank and observe the behavior of the “hail” in the apparatus as it is alternately attracted to and repelled from the plates.
f. Franklin’s Bells. After explaining the behavior of “Volta’s hail storm”,
build a Franklin’s Bell set-up. You need two empty soda cans, a wooden dowel, a large insulated platform to set the cans on (Styrofoam works well) and a string tied to the pop-top of one of the cans. Connect each terminal to a separate can, mounting the cans close together with the pop-top suspended between them (see picture). Turn the crank and observe the behavior of the suspended pop-top. Ben Franklin used one of these devices connected to his house roof so that he would be alerted when the air was highly-charged.
g. Charge a Leyden Jar with your Wimshurst Machine! (Be careful as this demonstration can be very dangerous!) Connecting the top terminal to the Wimshurst Machine, place the Leyden Jar on an insulated surface and crank the machine for a minute. After charging, discharge the Leyden Jar with a pair of insulated tongs and observe the “thick, white” spark produced. Students will witness an impressive spark of high voltage AND higher current than the Wimshurst alone. DO NOT allow ANY student to touch the Leyden Jar when it is charged!
Part 3 – The Tesla Coil
A Tesla Coil is a type of transformer invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity, stepping up ordinary 110-volt electricity to 10,000 to 50,000 volts. Tesla used these coils to conduct innovative experiments in electrical lighting, phosphorescence, x-ray generation, electrotherapy, and the transmission of electrical energy without wires for broadcasting, and the transmission of electrical power.
The YouTube video uses the “Super Mario Brothers” theme in stereo and harmony on two coils and shows a performance on matching solid state Tesla coils operating at 41 kHz. The device has been named the Zeusaphone, after Zeus, Greek god of lightning, and as a play on words referencing the Sousaphone.
Versions of the Tesla Coil are widely used as igniters for high power gas discharge lamps, common examples being the mercury vapor and sodium types used for street lighting. Blue-violet plasma filaments produced from a Tesla Coil can be seen as a result of the ionization of air due to the high voltage from the Tesla Coil. More familiar to students is the “Plasma Globe science toy” which uses a low power variation of the Tesla Coil to ionize gases within the globe. Tesla Coils have been used in “Star Trek” movies for special effects and used a glass-plate-type of plasma globe in the most recent “star Trek” film.
Using Your Tesla Coil
Although a Tesla Coil produces relatively high voltages associated with low currents, it is not advisable to allow students to take direct shocks from these devices. While the voltage is high, the current is low, and since high-frequency currents travel almost entirely at the surface of a conductor, the current does not produce much of a shock when passing through the body. However it will burn the skin at the point where the spark strikes it. Any heart electrical abnormalities could be affected by the coil. It is safe, however, for simple experiments in the classroom to illustrate the effects of high voltages.
a. A Tesla Coil will generate sparks and corona of
course, because of the high voltage created. Place a fluorescent light tube held in one hand or on a lab table will light when the high-frequency spark jumps to one end of it with no wires. Even a burned-out fluorescent light tube will glow when the Tesla Coil is held nearby.
b. A 300- to 500-watt incandescent light bulb
produces a very beautiful effect when the Tesla Coil is placed on a lab table in the dark and placed so the spark jumps to the base of the bulb. This effect is similar to the commercially-made plasma globes that are popular in science catalogs and novelty stores. The gas within the bulb becomes ionized by the high voltage and characteristic blue plasma streamers are generated within the bulb from the wires and bulb filament.
c. Small neon bulbs or gas tubes will glow with their characteristic color when a spark is generated. This is a great example of spectroscopy and how each gas exhibits its signature color when excited by a high voltage.
d. Two parallel stiff copper wires can be positioned
vertically and bent so that they are separated slightly as the wires rise, a “Jacob’s Ladder” can be constructed. As the Tesla Coil is touched to the wires, a violet spark is generated across the gap between the two wires and will rise due to the heating of the air as the spark is produced. This effect has been used for years in monster B-movies as “mad scientist” lab equipments. It is a great effect to demonstrate to your students. Wear a crazy wig and play a little horror movie music to add to the effect!
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