Assertion: tube's filament | /r/HasProperty | heated to 2500 °f
Facets:
value: to produce sufficient thermionic emission
type: PURPOSE
Source sentences:
text: The untreated thoriated tungsten filaments used in early vacuum tubes (called "bright emitters") had to be heated to 2500 °F (1400 °C), white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature, so they only have to be heated to 800–1100 °F (425–600 °C).