thumb|A plasma ball with filaments extending between the inner and outer spheres
A plasma globe, plasma ball, or plasma lamp is a clear glass container filled with noble gases, usually a mixture of neon, krypton, and xenon, that has 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 balls were popular as novelty items in the 1980s. Tesla called his invention an "inert gas discharge tube". The modern plasma lamp design was developed by James Falk and MIT student Bill Parker.
A crackle tube is a related device filled with phosphor-coated beads.
Construction
Although many variations exist, a plasma ball 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.
Plasma balls are driven by high-frequency (approximately ) alternating current at . is more easily polarized than the dielectric material around the electrode (i.e. the gas within the ball) 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 ball 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 thinner because the magnetic fields around it, augmented by the now-higher current flowing through it, cause a magnetohydrodynamic effect called pinch: the plasma channel's own magnetic fields create a force acting to compress the size of the plasma channel itself.
thumb|left|A "Tesla ball" at the [[NEMO (museum)|NEMO science museum in Amsterdam]]
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 ball, 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 .
An electric current is produced within any conductive object near the orb. The glass acts as a dielectric in a capacitor formed between the ionized gas and the hand.
Gallery
By adjusting the voltage, frequency, chemical composition and pressure of gas in the globe, many colorful effects can be achieved
History
thumb|Video of plasma ball
In ("Incandescent Electric Light", 1894 February 6), Nikola Tesla describes a plasma lamp. This patent is for one of the first high-intensity discharge lamps. Tesla used an incandescent-type lamp ball with a single internal conductive element and excited the element with high voltage currents from a Tesla coil, thus creating the brush discharge emanation. He gained patent protection on a particular form of the lamp in which a light-giving small body or button of refractory material is supported by a conductor entering a very highly exhausted ball or receiver. Tesla called this invention the single terminal lamp, or, later, the "Inert Gas Discharge Tube".
The technology needed to formulate gas mixtures used in today's plasma spheres was not available to Tesla. Modern lamps typically use combinations of xenon, krypton and neon, although other gases can be used.
Plasma balls can be used for experimenting with high voltages. If a conductive plate or wire coil is placed on the ball, capacitive coupling can transfer enough voltage to the plate or coil to produce a small arc or energize a high voltage load. This is possible because the plasma inside the ball and the conductor outside it act as plates of a capacitor, with the glass in between as a dielectric. A step-down transformer connected between the plate and the ball's electrode can produce lower-voltage, higher-current radio frequency output. Careful earth grounding is essential to prevent injury or damage to equipment.
Hazards
Bringing conductive materials or electronic devices close to a plasma ball may cause the glass to become hot. The high voltage radio frequency energy coupled to them from within the ball may cause a mild electric shock to the person touching, even through a protective glass casing. The radio frequency field produced by plasma balls can interfere with the operation of touchpads used on laptop computers, digital audio players, cell phones, and other similar devices.
See also
- Fusor
- List of light sources
- Sulfur lamp
- Vacuum arc