Christmas lights are a string of light bulbs used for decoration, usually around Christmas time but occasionally on other holidays like Halloween (using different colors and bulb shapes), although they can be found all year round in some places. They are commonly called Christmas lights because they are most often used as Christmas decorations. They are available in many colors, achieved by painting the bulbs or using colored glass, and in many shapes.
Christmas lights are most traditionally wrapped around the branches of a Christmas tree, but can also be found draped over fireplace mantles, encircling windows, draped over bushes, and even outlining an entire house. Lights intended for use on a Christmas tree have green cords to blend in with the green tree, ensuring the attention is on the light and not the cord. Net lights are arranged in a grid pattern to easily drape over and remove from bushes.
Lights come in indoor and outdoor versions. The outdoor lights are slightly more expensive but are weather resistant and approved for outdoor use by Underwriters Laboratories. Many upper middle class neighborhoods in the United States have friendly and informal competitions each year to see who can put up the most elaborate and best looking displays. These can get quite impressive and has spawned a market for professional decorators. It can also snarl traffic at night due to all the people driving very slowly around the neighborhood to see the displays.
Pagan Roots and History
Many customs and symbols in Christianity, especially Catholicism, were borrowed from the pagan religions of Northern Europe. As Christianity spread northward, they found it easier to absorb the pagan peoples into their religion if they brought bits of their old religion along with them. Thus the Christmas tree is evergreen as a symbol of everlasting life, because it does not go dormant in the winter months. Likewise Christmas lights were borrowed as a symbol of the returning sun as the days were getting longer after the solstice. Christmas was even put where it is on the calendar to replace the pagan ceremonies which took place around that time. The symbolism was just tweaked a little to adapt it.
For this reason, Christians decorate evergreen trees with lights to celebrate the day of Christ's birth, which most certainly did not occur on December 25th. Until the early 1900s, the trees were decorated with the only light source available: candles.
This of course posed a bit of a fire hazard. Not long after the Edison light bulb was invented, candles were beginning to be replaced by much safer electric lights. By the end of World War II, this transition was for all practical purposes complete. The 1970s saw another transition, from parallel wiring to series wiring.
Parallel vs. Series wiring
When I was young and learning the basics of how electricity works in grade school, I used to wonder why Christmas lights were wired in series. Most everyone is familiar with the old "one light goes out, they all go out" problem which results from this. Before shunts (explained below) were added to the lights, people had to go through the whole string to try to find the one burned out bulb and replace it. Wouldn't it be easier to wire it all up in parallel so only the burned out bulb would go out?
Parallel wiring
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~ O O O
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It turns out that before the 70s, they were indeed usually hooked up in parallel. The problem with this method was that each light bulb received 120 volts of electricity from the electrical outlet. That's a lot of juice for a little bulb. These lights burned very hotly and consumed a lot of power, and were a bit of a fire and safety hazard because of that, not to mention expensive to keep lit. But at least the rest kept burning when one went out.
Series wiring
+-----O-----O--...--O-----+
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~ O
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+-----O-----O--...--O-----+
In the 70s Christmas lights switched over to a series wiring method. Modern incandescent Christmas lights are very small, low wattage bulbs that run on only 2.5 Volts. This is much safer because the bulbs consume much less power each and therefore do not burn as hotly. They only feel warm to the touch. This is why strings of bulbs come in multiples of fifty (120V ÷ 2.5V/bulb = 48 bulbs, and rounding up makes little difference). Additional strings of 50 are added in parallel to each other, which is why most strings have three wires instead of only two.
Unfortunately this means that when one light bulb burns out, the electrical circuit is broken and the whole string of 50 goes out, making it difficult to find out which one caused the problem.
Blinking Lights
This did, however, lead to an interesting innovation. By installing a bimetalic strip in just one light on the string, the whole set of 50 can be made to blink on and off. As the electrical current powers the bulb, the bimetalic strip heats up. Because the two sides (bimetalic — two different metals) of the strip have different coefficients of thermal expansion, the strip bends as it heats up until it loses contact with the circuit, shutting off the whole string. As it cools, it bends back to make the connection again, and the process repeats to cause the set to blink. Fancier modern sets have solid state microchips and transistors instead, which can cause the set to blink in interesting patterns instead of a simple on/off.
Finding the Bad Bulb
There are a few different ways to find the bad bulb in the string. Most people pull out and replace each bulb, starting from one end and working their way up the string, until the lights come on again. They then throw away the last bulb they replaced. However, if the bulb they are using to do the replacements is burned out itself, or if there are two burned out bulbs in the string, this will only lead to a great deal of frustration and wasted time.
Another method is to test the bulbs. A 9 volt battery can be used for this by touching the wires at the base of the bulb to the terminals of the battery, conveniently spaced just about right for this purpose. However the 9 volt battery provides almost four times more than the bulb's rated voltage so don't leave it hooked up too long or you could shorten the bulb's lifespan.
A third method is by using a non-contact voltage tester wand. These are small, battery powered devices shaped like a thick marker which detect electric fields, which are generated by the presence of alternating current voltage. Since the voltage stops at the burned out bulb, the tester can be used to find the burned out bulb by passing it up along the string until it stops detecting voltage. The last bulb it passed successfully is the one burned out. These testers are actually the same kind used by electricians to test if circuits are live before working on them, just less rugged and packaged in Christmas colors.
Shunt Bulbs
These methods are time consuming and it's often easier to throw away the $5 string of lights and buy a new one. The Christmas light industry has heard your cries of despair, however, and has provided a better solution. Most miniature light bulbs these days come with a device called a shunt in parallel with the filament. The shunt is a high resistance connection which does not interfere with the operation of the filament because very little current passes through the high resistance of the shunt compared with the low resistance of the filament.
When the filament burns out, all the current passes through the shunt instead. This burns the high-resistance coating off the shunt and turns it into a good conductor of electricity, which maintains the circuit and keeps the rest of the string lit. From there all you have to do is pull out the one dead bulb and replace it. However, since the shunt is in the bulb, it only works while the bulb is plugged in, and the string will still fail if the bulb is merely loose.
Fused Plug
It should be mentioned though that all of these methods will fail if the problem is that the fuse in the plug at the electrical outlet has blown. Since the string of lights is so small and low power, this fuse helps protect them from the mighty 120 VAC power source in your wall. Otherwise a short-circuit somewhere in the line could set the tree on fire, significantly reducing one's holiday merriment. While the fused plug is a ubiquitous device in UK electrical cords, outside of Christmas lights they are almost unheard of in the US.
The plug will also often have an extension outlet built-in so you can plug another set of lights into that set to reduce the number of outlets needed.
Toys
Because the string of lights is just a series of electrical plugs with bulbs in them, manufacturers have begun selling small, low power toys that plug into them. These can do any of a wide number of things, such as move with very small motors, or play a recorded message, or flash LEDs on and off. Everything from animated reindeer to Star Trek spaceships that flash lights and play holiday messages from Leonard Nimoy is available.
Of course, simpler ornaments that merely fit around the bulb so that they glow are also common. These are popular for making Halloween theme lights with glowing witches and pumpkins. Christmas themes include icicles, snowmen, and, looking back to the olden days, artificial candles.
Other Types
Although incandescent bulbs dominate the market because they're so cheap, a few other technologies are available.
LEDs last longer and use less power, but most require a transformer and rectifier to operate at low DC voltage rather than run in series.
Neon bulbs run at full wall voltage so they are wired in parallel, and are much safer than incandescents wired this way.
Artificial trees sometimes come with fiber optic lines permanently installed in the branches. These can be made to change color with a rotating disc filter over the light source in the base of the tree.