1705,1894

Gas-discharge lamp

• Francis Hauksbee knew the empty space
• in a thermometer could glow when it was handled,
• so he put a little mercury in a glass globe,
• pumped out some of its air,
• rubbed and touched the globe, and found
• it emitted a light bright enough to read by.

Plasma lamp

• In 1894, Nikola Tesla invented
• a lamp without an internal electrode.
• He subjected an evacuated glass tube
• to a high-frequency alternating current,
• starting an avalanche of electrons
• causing excited atoms to emit photons.

Did he

• Did Tesla know what he was doing?
• Does it matter? Did he know what he had found?
• Did he know he didn’t need high voltage;
• he didn’t need alternating current?
• Obviously he knew about the light, but
• three years later Thomson discovered electrons
• and Townsend discovered electron avalanches.

Gas discharges

• Saint Elmo’s fire,
• mercury gas lit by static,
• Geissler and Crookes tubes,
• Geiger counters,
• fluorescent lamps, neon,
• metal halide, mercury vapor,
• deuterium arc, xenon arc,
• sodium, and plasma lamps.