Thermionic emission On a Relation between Heat and Static Electricity Edison effect Schottky effect Thermion dance Commentary

Frederick Guthrie, Thomas Edison electromagnetism

Thermionic emission

• Heating a cathode frees electrons,
• which flow through a vacuum
• to an anode with a positive charge
• but which do not flow
• to an anode with a negative charge
• even if you provide to the cathode
• new electrons from a battery.
• Today this is called
• thermal electron emission.
• At the time, the electron
• had not been discovered.
• They knew the hot cathode
• emitted something,
• so they called it a thermion.

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On a Relation between Heat and Static Electricity

• Frederick Guthrie’s experiments in 1873
• tested how a hot cast-iron ball
• carried positive or negative
• static electrical charges
• and discovered the ball would not carry
• a positive charge when it was hot
• and the ball would carry a negative charge
• only when it was hot.

Edison effect

• In 1880, Thomas Edison,
• or William Joseph Hammer,
• Edison’s assistant when
• developing the light bulb,
• added a lead to a metal foil
• in a few light bulbs
• to see why the inside
• of light bulbs blackened
• near the positive terminal
• of the hot filament,
• and found that a current
• flowed from the filament
• when the added foil
• had a positive charge
• but not when it
• had a negative charge.

Schottky effect

• The emission of electrons from a hot surface
• is increased by applying an electric field
• to hold a negative electric charge at the surface.

Thermion dance

• A metallic surface in a vacuum
• is half metal, half vacuum, and dancing
• dancing on that surface, dance thermions.
• When the surface gets too hot
• thermions cannot stay on, and so they slip,
• they slip into the vacuum like dust motes.
• Thermions slip like drops of mist, float
• into space, drift like helpless mosquitoes,
• and fly like tiny moths attracted to a lamp.