Icicle lights. LEDs. It seems like every few years there’s another “big thing” in Christmas lighting. But how do they all work?
General Electric’s first sets of pre-wired lights intended for home Christmas trees came out in 1904-1905. (Edward Johnson, a business associate of Thomas Edison, was the first to use electric lights on a Christmas tree in 1882, but for many years all such displays were hand-wired.) Early lights were round, but the popular cone shape took over by 1922.
In1927, General Electric introduced new, larger lights for outdoor use and, along with the electric companies, began sponsoring neighborhood decorating contests to encourage their use. The idea, needless to say, caught on.
Miniature lights were first produced in Italy and introduced to North America around 1950. Flashing lights, introduced in Japan, followed about 1955. Today, miniature lights are the lights of choice for most Christmas trees.
The original strings of Christmas lights went dark whenever even one bulb burned out because they were wired in series–the electricity had to flow through each bulb to get to the next. The earliest outdoor lights, and later indoor lights, were wired in parallel: each bulb is part of its own separate circuit, and thus the loss of one does not interrupt electricity to the others.
Miniature lights are also wired in series, but they don’t go dark when a bulb burns out (although they do if you pull out a bulb). That’s because most mini-light bulbs contain a special shunt wire below the filament, coated with a high-resistance substance. When the much lower-resistance filament burns out, the electricity is forced to go through the shunt, which heats up. That burns off the resistance coating. The rest of the string doesn’t notice that one bulb is no longer working.
There are a lot of mini-bulbs on a string because they’re only 2.5-volt bulbs, but they’re plugged into a 120-volt power outlet. Divide 120 by 2.5 and you get 48, and originally, that’s how many bulbs were on a miniature light string. Typical strands today have 50 bulbs. Very long strings of miniature lights simple string 50-bulb series in parallel (so only part of the string goes dark if you lose a bulb).
A simple blinker bulb sends current through a bi-metallic strip, made of two metals which expand at different rates when heated. When the filament gets hot, one side of the bi-metallic strip expands more than the other, causing the strip to bend away from a metal post, breaking the circuit. Once the strip cools, it uncurls, and the electricity flows again. An integrated circuit in a controller box can be programmed to turn two or more interwoven strands of mini-lights on and off at varying intervals to produce many more interesting (or annoying) effects.
Over the next few years, we may see all other forms of Christmas lights give way to the Next Big Thing: light-emitting diodes, or LEDs.
A diode is a simple semiconductor device. A semiconductor is a material—in an LED, aluminum-gallium arsenide–that sometimes conducts electricity and sometimes doesn’t. In pure aluminum-gallium-arsenide, there are no free electrons to conduct current. But if it is “doped” with impurities, it can be made into either an “N-type” material, with an excess of electrons, or a “P-type” material, with not enough electrons—with, in effect, empty holes where electrons should be.
In a diode, N-type material is bonded to P-type material. A negative electrode is attached to the N side and a positive to the P side. As long as no current is applied, the N-type’s electrons fill up the P-type’s holes along the interface between the two materials, and the diode is non-conducting. Apply a current, though, and the N-type’s electrons are repelled from the negative electrode and drawn to the positive electrode. As they move, they jump in and out of the holes in the P-type material. In the process, they change energy levels, and whenever an electron changes energy levels, it emits a photon—light. The color of the light is determined by the size of the change in energy levels.
LEDs emit very little heat along with their light, so they use only about a tenth as much energy as an equivalent incandescent string. They can also be encased in thicker plastic, which makes them essentially unbreakable. And they’ll probably last 20 years—maybe longer.
Unlike the floodlight-lit aluminum Christmas trees of my youth (remember those?), I think this is one Christmas lighting innovation that’s here to stay.
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If you’re interested in this topic, be sure to check out The Antique Christmas Light Site.
