سوال ۲۵ کنکور ارشد مهندسی برق ۱۴۰۰

PASSAGE 2 (excerpt):

Most sources of illumination generate light over an appreciable period, and indeed if an object is lit for a very brief time (less than 1/25 second), the human eye will not react in time to see the object. A photographic emulsion---that is, a light-sensitive coating on photographic film, paper, or glass---will, however, record much shorter bursts of light. A photographic flash can therefore be used to capture high-speed movement on film as well as to correct deficiencies of the normal surrounding lighting. Photoflash is now generated electronically, but the earliest form, first used in 1864, was a paper bag containing magnesium wire and some oxygen-rich substance, such as potassium chlorate. When the bag was ignited, the metal burned with an intense flash. A contemporary observer reported that "this quite unsafe device seems to have done nothing worse than engulf the room in dense smoke and lead to pictures of dubious quality and odd poses."

The evolution of the photoflash was slow, flashbulbs, containing fine wire made of a metal, such as magnesium or aluminum, capable of being ignited in an atmosphere of pure oxygen at low pressure, were introduced only in the 1920's. In the earliest type, the metal was separated from the oxygen by a thin glass bulb. The flash was fired by piercing the bulb and allowing the oxygen to come into contact with the metal, which ignited spontaneously. Later bulbs were fired by an electric battery, which heated the wire by passing a small current through it.

Question: Which of the following is defined in the passage?