Marekizer Plasma Globe

Introduction

Spectrum tubes, fluorescent lightbulbs, novelty “plasma globes” and glowing “neon” signs all have one thing in common—they contain a gas that glows a specific color when a high voltage is applied. In this demonstration, an inexpensive “plasma globe” will be prepared from a Tesla coil and a simple lightbulb.

Concepts

• Emission spectra
• Plasma globes
• Excited electron states
• Tesla coils

Materials

Safety Precautions

A Tesla coil produces high-voltage, low-current electricity at a very high frequency. The electric shock produced by the Tesla coil is minimal. However, presenters or students with medical conditions (e.g., heart conditions) that may be affected by high-voltage electricity should not operate or touch a Tesla coil. The electric sparks will produce a burning, tingling sensation on the skin. Do not spark the same area of skin for more than one or two seconds.

A small amount of ozone is produced by the sparks of the Tesla coil. Ozone is toxic by inhalation. Operate the plasma globe in a well-ventilated room. Do not touch the metal base of the lightbulb or the metal coil when the Tesla coil is turned on. A strong electric shock may result. Touch only the glass dome of the lightbulb when performing the demonstration. Caution students about the shocking sensation they will feel before they touch the plasma ball. The demonstrator and all observers should wear safety glasses.

Procedure

1. Screw the lightbulb in to the large-diameter opening of the Marekizer coil (see Figure 1a).
2. Slide the smaller-diameter opening of the Marekizer coil over the Tesla coil needle (see Figure 1b).
3. Plug in the Tesla coil and turn off the lights in the classroom.
4. Place your fingers on the lightbulb dome.
5. Turn on the Tesla coil and twist the adjustment knob on the Tesla coil until the lightbulb glows dimly and several plasma sparks can be seen inside the bulb arcing to your fingers.
6. A tingling sensation will be felt on the tips of your fingers. Slowly move your fingers around the globe, being careful not to break contact with the globe (an unpleasant shock may occur).
7. The sensation that is felt when touching the Marekizer is unusual and may also be painful and stunning to some students. The electric sparks produced by the Tesla coil are not harmful, however. Inform and caution students about the sensation they will feel if they decide to touch the Marekizer so that they will not be startled the first time they touch it. Students should not be required or forced in any way to touch the lightbulb if they are not comfortable doing it.

Teacher Tips

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  The Marekizer coil should have an indefinite life span if it is stored properly. The lightbulb has a limited life span but should last for at least fifty demonstrations. To improve the lifetime of the lightbulb, do not operate it continuously for more than a minute, and allow the lightbulb to cool between demonstrations. An extra lightbulb is provided as a replacement. Store the lightbulbs in paper towels to prevent breakage.
• Clear, incandescent replacement lightbulbs are available at most home-improvement or hardware stores. Experiment with various styles and sizes of unfrosted lightbulb styles. High-wattage lightbulbs glow brighter and have more visible electric arcs than low-wattage bulbs. Lightbulbs that are rated 40- or 60-W work well.
• To insulate the wire coil and base of the lightbulb in order to prevent electric shocks from these regions, place a polystyrene cup over the metal, as shown in Figure 2. Obtain an 8-oz polystyrene cup. Cut a ¾"-diameter hole (approximately) in the bottom center of the cup. Carefully insert and screw the wire coil into the hole until the lightbulb dome reaches the bottom of the cup. Use a small amount of transparent tape to secure the cup to the bulb, if necessary. This cup will prevent students from touching the metal coil (see Figure 2).
• Hold a fluorescent lightbulb in one hand while touching the operating Marekizer Plasma Globe with the other. Watch as the fluorescent lightbulb glows! This occurs because the high-frequency electricity travels over the surface of the body, instead of through it. The electricity from the Tesla coil travels over the surface of the body from one hand to the other, causing the fluorescent lightbulb to glow. If the electricity were only able to travel through the body, the internal resistance would greatly decrease the power output to the hand holding the fluorescent lightbulb.
• These demonstrations are most effective in a darkened room.

Discussion

Spectrum tubes consist of a gaseous element or compound in an evacuated glass tube equipped with metal electrodes. When a high voltage is applied across the electrodes, the gas molecules inside the tube interact with the high-energy electrons provided by a high-voltage power supply and absorb the electrons’ energy. The energy provided by the external electrons is strong enough to promote, or “excite,” the ground-state electrons of the gas molecules (see Figure 3). The molecular electrons “jump” from their ground state to various excited energy levels. A few moments later, through spontaneous emission, the electrons naturally “fall” from these excited energy levels back to a lower energy level, or all the way to the ground state. When the excited electrons relax, energy is released in the form of a photon of light. The wavelength of the photon is dependent upon the energy that is released. The higher the energy change, the shorter the wavelength of light. If the photons have wavelengths in the visible portion of the electromagnetic spectrum (400–700 nm), then the glass tube will glow with a visible color that is characteristic of the gas inside the tube. Just as a fingerprint is unique to every individual, each element emits a characteristic color pattern of light after excitation.

Most high-wattage (>60-W), incandescent lightbulbs are filled with a mixture of argon and nitrogen gas under a lower than atmospheric pressure. When the high-voltage electrons pass through the bulb, the electric sparks extending from the filament (electrode) glow with a lavender or bluish color. This color is characteristic of the visible glow produced by an argon-nitrogen spectrum tube. The low pressure inside the bulb favors long electric arcs that dramatically extend from the filament to the grounded glass dome. Under normal operating conditions, the lightbulb emits a “white light” and not blue- or lavender-colored light. This is because the heated tungsten filament emits photons that cover a broad range of wavelengths in the visible spectrum. The broad range of “colorful” wavelengths blend to form “white light.”

A Tesla coil provides high-frequency, high-voltage electricity at a low current. In a Tesla coil, the high-voltage “recharges” approximately 30,000 times or more per second. This rapidly repeating cycle of high voltage maintains the proper excitation energy for the gas molecules in the glass tube.

References

Special thanks to Lee Marek, University of Illinois–Chicago, Chicago, IL, for providing the idea and instructions for this activity to Flinn Scientific.