Bright Emitters

The first valves (tubes) from 1905 were very like light bulbs in construction of the glass envelope (tube or bulb), connection base, seal (pinch) and filament. The pure tungsten filament had to be heated to 1400°C (2500°F) to get sufficient electron emission. The oxide coated "dull emitter" might only need 450°C (900°F). The use of Thorium alloyed to the Tungsten was discovered in 1914 and made practical by Irving Langmuir in 1923 (He also made substantial development to electric lamps). This reduced the temperature (and thus brightness and power) increasing life and is called a thoriated tungsten filament. It's still used for very high power transmitting tubes.
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