A plasma ball, 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 (see corona discharge and electric glow discharge). Plasma balls were 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 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 . 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 ball through an electrode at its center. Additionally, some designs utilize the ball 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 ball (see corona discharge and electric glow discharge). If a hand is placed close to the ball it produces a faint smell of ozone, as the gas is produced by high voltage interaction with atmospheric oxygen. Some balls have a control knob that varies the amount of power going to the center electrode. At the very