Many lights use sensors such as buried wire loops, cameras or radar to detect vehicles, while others simply follow a preset timer. A controller combines both to decide when to change.
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Roman concrete combined volcanic ash, lime and self-healing mineral growth, and it had no steel to corrode. Researchers still debate which factor matters most.
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Microwaves make water molecules in food rotate and heat up. Metal sparks when microwaves push charge onto sharp edges or points, creating an electrical arc.
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Containers cut shipping costs and lifted trade, but they favored ports with deep water and open land, so older city-center ports declined while newer ones rose.
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A pin tumbler lock uses spring-loaded pin pairs that block a rotating plug until the right key lines them up. The idea echoes ancient Egyptian locks; Linus Yale Jr. shaped the modern form.
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A flush toilet releases tank water into the bowl, which fills a curved trapway and starts a siphon that pulls waste out. Air then breaks the siphon and the tank refills.
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Elisha Otis invented the elevator safety brake in 1852, not the elevator itself. Modern elevators add layered electrical and mechanical safeguards, including many cables, governors and buffers.
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Smoke detectors sense airborne particles, mostly with ionization chambers that register a drop in electric current or photoelectric chambers that register scattered light.
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Grid operators forecast demand, watch system frequency, and instruct power plants and other providers to raise or lower output, using automatic controls for seconds and markets for minutes to hours.
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Water towers store treated water high above town. Pumps fill them when demand is low, and gravity then supplies pressure and water at busy times or during outages.
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The deck hangs from curved cables in tension. The cables pass over towers that carry the load down in compression, and anchorages at each end hold the cables' pull.
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A quartz watch counts the very fast, steady vibrations of a small quartz crystal. A circuit divides them down to one pulse per second, which drives a small motor that moves the hands.
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A coil under the glass sends a rapidly changing magnetic field into the pan, which induces electric currents in the metal. The pan's resistance turns those currents into heat.
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Utilities typically clump and settle out particles, filter the water, and disinfect it. The exact steps depend on whether the source is surface water or groundwater.
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An escalator is a loop of linked steps pulled by motor-driven chains along tracks that keep each step level, with handrails driven in sync with the steps.
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The zipper grew from 19th-century fastener patents and became a practical device when Gideon Sundback redesigned it around 1913. Its interlocking teeth are locked and unlocked by a Y-shaped slider.
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