Plasma lamp

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A plasma lamp.

Plasma lamps (also variously plasma globes, balls, domes, spheres, tubes or orbs) are novelty items which were most popular in the 1980s. The plasma lamp was invented by Nikola Tesla after his experimentation with high frequency currents in an evacuated glass tube for the purpose of studying high voltage phenomena, but the modern versions were first designed by Bill Parker.[1] Tesla called this invention an inert gas discharge tube.


[edit] Description

The effect of a conducting object (such as a hand) in close proximity with the plasma globe glass.

Most commonly, plasma lamps are available in spheres and cylinders. Although many variations exist, a plasma lamp is usually a clear glass orb, filled with a mixture of various gases—most commonly helium and neon, but sometimes also xenon and krypton at low pressure (below 0.01 atmospheres)[2]—and driven by high frequency alternating current at approximately 35 kHz, 2–5 kV, generated by a high voltage transformer. A much smaller orb in its center serves as an electrode. Plasma filaments extend from the inner electrode to the outer glass insulator, giving the appearance of multiple constant beams of colored light (see corona discharge and electric glow discharge). The beams initially follow the electric field lines of the dipole, but move upwards due to convection.[citation needed]

Placing a hand near the glass alters the high-frequency electric field[citation needed], causing a single beam to migrate from the inner ball to the point of contact. An electric current is produced within any conductive object near the orb, as the glass does not block the electromagnetic field created by the electric current flowing through the plasma (though the insulator does block the current itself).[citation needed] The glass acts as a dielectric in a capacitor formed between the ionized gas and the hand.

[edit] Potential hazards

Interior of a plasma ball.

Caution should be made when placing electronic devices (such as computer mice) near or upon the plasma lamp: not only may the glass become hot, but the high voltage may place a substantial static charge on the device, even through a protective plastic casing. The radio frequency field produced by plasma lamps can interfere with the operation of touch pads used on laptop computers, digital audio players, cell phones, and other similar devices. Some types can radiate sufficient RFI to interfere with cordless telephones and Wi-Fi devices several feet away. Additionally, when a metal object (such as a coin) is placed on the surface of a plasma lamp's glass, a danger of shock and burning exists; it is very easy for electricity to be emitted from the lamp if the metal comes in contact or proximity with certain other materials, including human tissue. If a medium-sized lamp is wrapped in grounded metal foil, capacitive coupling can transfer tens of milliamperes to ground through the foil: enough to light a small lamp or give a small arc burn. This is possible because the glass acts as a capacitor dielectric: the inside of the lamp acts as one plate, and any conductive object on the outside acts as the other capacitor plate.

In addition, it is possible (although difficult) to cause a minor burn to the finger by hovering the fingertip just above the surface of the glass. This causes a small arc to form and generate heat by simultaneously "optimising" both the voltage across the gap and the current through it.

Ozone, which is harmful to humans, may also accumulate far outside of the surface of the glass orb after a few minutes of constant operation. It accumulates at a higher rate if a hand or metal object is placed on the glass.

Since the device also releases electromagnetic radiation, a person with a pacemaker should not touch the device, due to a possibility of pacemaker failure.

[edit] History

Plasmaball vid2.ogg
A plasma lamp in action.

In U.S. Patent 0,514,170  ("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 took an incandescent type lamp globe with the suspended conductive element and excited the element with high voltage currents from a Tesla coil, thus creating the brush discharge emanation. He gains patent protection on the particular forms 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 globe or receiver. Tesla later called this invention an "Inert Gas Discharge Tube".

The popular product sold throughout the world today was invented by Bill Parker while an undergraduate student at MIT in 1970.

The technology needed to formulate gas mixtures used in today's plasma spheres, primarily combinations of high purity rare gases, was not available to Tesla. These gas mixtures, glass shapes and integrated circuit driven electronics used to create the vivid colors, range of motions and complex patterns seen in today's plasma spheres were all developed and patented by Bill Parker in the 1980s and 1990s. The lamps typically contain xenon, krypton and/or neon, though a number of other gases can be used as well.

[edit] Applications

Plasma lamp running. Note when the finger touches the bulb.

Plasma lamps are mainly used as curiosities or toys for their unique lighting effects and the 'tricks' that can be performed on them by moving the hands around them. They might also form part of a school's laboratory equipment for demonstration purposes. They are not usually employed for general lighting. However as of recent years some novelty stores have begun selling a night light plasma lamp that can fit into any standard light socket.

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