Human beings like light. We don’t see well at night, and our vaunted intelligence goes hand in hand with a vivid imagination that loves to populate shadows with Things That Go Bump in the Night.
As a result, we’ve always looked for ways to light up our lives: campfires, torches, candles, oil lamps, gas lamps…and, as of last century, electric lights.
The first electric light was the electric arc lamp, invented around 1801 by the British chemist Sir Humphry Davy. This consisted of two carbon electrodes set just far enough apart that electricity could jump from one to the other. Air conducts electricity very reluctantly, getting very hot as it does so and giving off lots of angry blue-white light. In 1862, an electric arc lamp was installed in a lighthouse in Dungeness, England. Unfortunately the carbon electrodes burned out quickly and had to be replaced every few hours.
Davy also experimented with passing a current through a fine platinum wire, causing it to get hot and glow. This “incandescent” form of lighting seemed promising, but nobody came up with a practical design until the American inventor Thomas Alva Edison took up the problem.
One problem with incandescent lamps was that the filament burned away very quickly. Edison’s solution was to encase the filament in a glass bulb from which the air had been evacuated. In the absence of oxygen, the filaments of carbonized bamboo he came up with glowed white-hot for up to 40 hours.
Patented in 1879, the Edison light bulb took the world by storm. Most houses still use a version of it today, but it’s undergone some refinements.
In 1907, carbon filaments were replaced with tungsten filaments, which lasted longer. However, evaporating tungsten blackened the inside of the bulb. In 1913 the vacuum in light bulbs was replaced with a mixture of nitrogen and argon, which slowed this evaporation, keeping the glass from blackening and making the filament last longer. A modern 75-watt bulb has a life expectancy of 750 hours.
The latest version of the incandescent light bulb is the halogen light, filled with iodine or bromine, which re-deposit evaporating tungsten onto the filament. Halogen bulbs last up to 3,000 hours and produce a whiter light, because the tungsten can be heated to a much higher temperature. There are two drawbacks to this: the bulb gets dangerously hot and it produces unhealthy amounts of ultraviolet radiation. To counteract this, halogen lights are encased in a quartz bulb inside an outer glass shell.
The principle behind fluorescent lights–that passing an electrical current through certain gases could cause them to glow–was discovered in the 1860s, but it took many years of experimentation to make it practical. Fluorescent lighting was exhibited at the Chicago Centennial Exposition in 1933, and by the 1950s had largely replaced incandescent lighting in public buildings, mainly because it’s more efficient.
Only five percent of the energy that goes into an incandescent light bulb produces light; the other 95 percent produces heat. Fluorescent lights emit more light per watt.
An ordinary fluorescent light is filled with argon and a small amount of mercury vapor. A “starter” pushes electricity through electrodes at each end of the light tube. This electricity begins ionizing the gas in the tube–giving the molecules an electrical charge by knocking away electrons. When enough of the gas is ionized, electricity begins to flow through the gas itself, which glows. Most of the glow is actually invisible ultraviolet light, so the inside of the tube is coated with a phosphor, a substance that glows under ultraviolet light, and it’s that glow that we see. By changing the composition of the phosphor, you can change the colour of the light.
Fluorescent lights are really just one type of “electric-discharge” light which produce illumination by passing a current through an ionized gas. Others are the neon light; the mercury-vapor arc lamp (the archetypal streetlight); and the sodium-vapor light, with its bright, distinctive yellow-orange glow.
Since the dawn of time, humans have been trying to banish darkness from their dwellings, their places of business, and even their streets. Thanks to electricity, we’ve succeeded to a remarkable degree, but some things never change: we’re still afraid of Things That Go Bump in the Night.
The only difference is, these days we figure it’s a burglar instead of a bogeyman.
