page_title: Plasma globe
section_title: Description
hierarchical_section_title: Plasma globe / Description
caption_attribution_description: Nederlands: Teslabol, 1999 Teslabol
context_page_description: A plasma globe or plasma lamp is a clear glass container filled with a mixture of various noble gases with a high-voltage electrode in the center of the container.
  When voltage is applied, a plasma is formed within the container. Plasma filaments extend from the inner electrode to the outer glass insulator, giving the appearance of multiple constant beams of colored light. Plasma globes were most popular as novelty items in the 1980s.
  The plasma lamp was invented by Nikola Tesla, during his experimentation with high-frequency currents in an evacuated glass tube for the purpose of studying high voltage phenomena. Tesla called his invention an "inert gas discharge tube". The modern plasma lamp design was subsequently developed by Bill Parker, a student at MIT.
context_section_description: Although many variations exist, a plasma lamp is usually a clear glass sphere filled with a mixture of various gases (most commonly neon, sometimes with other noble gases such as argon, xenon and krypton) at nearly atmospheric pressure. A crackle tube is a related device filled with phosphor-coated beads. Plasma lamps are driven by high-frequency (approximately 35 kHz) alternating current at 2–5 kV. The drive circuit is essentially a specialized power inverter, in which current from a lower-voltage DC supply powers a high-frequency electronic oscillator circuit whose output is stepped up by a high-frequency, high-voltage transformer. The radio-frequency energy from the transformer is transmitted into the gas within the globe through an electrode at its center. Additionally, some designs utilize the globe as a resonant cavity, which provides positive feedback to the drive transistor through the transformer. A much smaller hollow glass orb can also serve as an electrode when it is filled with metal wool or a conducting fluid that is in communication with the transformer output. In this case, the radio-frequency energy is admitted into the larger space by capacitive coupling right through the glass. Plasma filaments extend from the inner electrode to the outer glass insulator, giving the appearance of moving tendrils of colored light within the volume of the globe (see corona discharge and electric glow discharge). If a hand is placed close to the globe it produces a faint smell of ozone, as the gas is produced by high voltage interaction with atmospheric oxygen.
  Some globes have a control knob that varies the amount of power going to the center electrode. At the very lowest setting that will light or "strike" the globe, a single tendril is made. This single tendril's plasma channel engages enough space to transmit this lowest striking energy to the outside world through the glass of the globe. As the power is increased, this single channel's capacity is overwhelmed and a second channel forms, then a third, and so on. The tendrils each compete for a footprint on the inner orb as well. The energies flowing through these are all of the same polarity so they repel each other as like charges: a thin dark boundary surrounds each footprint on the inner electrode.
  Placing a finger tip on the glass creates an attractive spot for the energy to flow, because the conductive human body (having non-ohmic resistance of about 1000 ohms at room temperature) is more easily polarized than the dielectric material around the electrode (i.e. the gas within the globe) providing an alternative discharge path having less resistance. Therefore, the capacity of the large conducting body to accept radio frequency energy is greater than that of the surrounding air. The energy available to the filaments of plasma within the globe will preferentially flow toward the better acceptor. This flow also causes a single filament, from the inner ball to the point of contact, to become brighter and thinner. The filament is brighter because there is more current flowing through it and into the 150 pF capacity, or capacitance, presented by an object, a conducting body, the size of a human. The filament is thinner because the magnetic fields around it, augmented by the now-higher current flowing through it, causes a magnetohydrodynamic effect called self-focusing: the plasma channel's own magnetic fields create a force acting to compress the size of the plasma channel itself.
  Much of the movement of the filaments is due to heating of the gas around the filament. When gas along the filament is heated, it becomes more buoyant and rises, carrying the filament with it. If the filament is discharging into a fixed object (like a hand) on the side of the globe, it will begin to deform into a curved path between the central electrode and the object. When the distance between the electrode and the object becomes too great to maintain, the filament will break and a new filament will reform between the electrode and the hand. (See also Jacob's Ladder, which exhibits a similar
caption: A "Tesla ball" at the NEMO science museum in Amsterdam
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