1901

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.