Ferdinand Kurlbaum,
Ludwig Holborn,
Harmon Northrup Morse
thermometry
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Disappearing-filament pyrometer
- A hot filament
- at the focal plane
- of the objective lens
- of a Keplerian telescope,
- given a line of sight
- to a glowing hot object,
- was the first device
- that could measure
- temperatures
- over 1000°C.
- According to the rule
- that two objects
- at the same temperature
- emit the same
- black-body spectrum,
- the filament and the object
- have the same temperature when,
- focused on the hot object,
- and adjusting the current
- that heats the filament,
- the filament seems
- to disappear.
Ideal emissivity
- An ideal black body emits
- the same energy
- at the same frequency
- as any other ideal black body
- in thermal equilibrium
- at the same temperature.
- Other than that, everything’s
- an imperfect black body,
- emitting radiation all the time,
- unless it’s as cold as absolute zero.
- You just can’t necessarily
- see its radiation.
Disappearing act
- Alone in a darkened room,
- as in a box that reflects no light,
- you might see the heat rising
- from an extinguished candle,
- or a light-show moving slowly
- when you shut your eyes tightly.
- Although the room
- has disappeared,
- other senses tell you
- you remain in its space.
- When they left him alone
- in a sound-proof room,
- John Cage was alarmed
- by the rushing of his blood.
- There’s no reason to panic;
- nothing is gone except the light.
- Reason alone tells you
- the rabbit is back in the hat.
The disappearing-filament pyrometer is great for measuring the temperatures of objects that are so hot they glow, but it was not the first to determine temperatures at a distance. That honor goes to Langley’s bolometer.
See also in The book of science:
Readings in wikipedia:
Other readings: