The Enlightening Science of Christmas Lights

It’s the most wonderful time of the year again—yes, the Christmas season! As you count down the days until Christmas, you have probably been preparing yourself for the best holiday of the year. You might have been making your Christmas wish list or deciding on the gifts you will give your loved ones. Maybe you have been decorating your house and even putting out a Christmas tree! Whatever you have or have not done in preparation for the holidays, you are certainly bound to hang up your Christmas lights at some point. But have you ever wondered how Christmas lights—which light up your house and your spirits—actually light up?

Christmas lights were invented in 1882 by Edward Hibberd Johnson, who worked for Thomas Edison, the man who discovered the light bulb. After seeing his work, Johnson strung 80 red, white, and blue light bulbs he hard-wired around a Christmas tree.
Companies specializing in holiday lights no longer hard-wire light bulbs to create Christmas lights. Instead, these lights are placed into a machine where they undergo a few processes before they come out as the colorful lights we love.
There are currently two kinds of Christmas lights: the traditional ones which use filaments and the LED lights which use light-emitting diodes. Both are wired in a series-parallel arrangement. However, traditional lights have shunts and fuses which LED lights do not have.

This story begins with a man living in a townhouse at 136 East 36th Street in New York City. This man’s name was Edward Hibberd Johnson, and one day, he had an idea. Johnson worked for famous inventor Thomas Edison, who we all know for his discovery of the light bulb, as a consultant for the Automatic Telegraph Company. Johnson was so impressed by Edison that he continued working for him even long after he went on and started a new company.

Just like any person who decorated their Christmas tree, Johnson wanted his tree to reflect light and joy, to reflect the spirit of Christmas. While candles were once responsible for giving Christmas trees their festive look, they were unfortunately also a fire hazard—and who would want to burn their house down during the holidays, right? Luckily for Johnson, he saw an opportunity, a solution even, at the Edison shop. Driven by the possibilities of applying electricity to Christmas, he set out to create holiday lights.

After stringing 80 red, white, and blue light bulbs and hand-wiring them around a Christmas tree, Christmas lights were officially born. Shortly after, he called W.A. Croffut, a veteran writer for the Detroit Post and Tribune, who was amazed by his lights. Croffut wrote, “At the rear of the beautiful parlors, was a large Christmas tree presenting a most picturesque and uncanny aspect. It was brilliantly lit with…eighty lights in all encased in these dainty glass eggs, and about equally divided between white, red, and blue…One can hardly imagine anything prettier.” Flash forward to a few centuries later, and his lights are cherished not only by one reporter but by millions of households.

Back in 1882, Johnson hand-wired the light bulbs and wound them around his Christmas tree; now, holiday light companies use a machine to create our beloved Christmas lights. They start by feeding the wire into a machine to be cut and shaped. Then, they strip the tip of the wire and coat it with flux. (Coating it with flux is essential for the next step as flux makes soldering and welding metals a bit more convenient!) While in the machine, the light bulbs are tested for positive or negative charges. Afterwards, they use the machine to line up all the stripped parts of the string with the light bulbs, effectively combining them. Once the wire and bulb are connected, each light is tested for an electric charge again. The metal piece between the light bulb and the string is then covered with an insulator. The string is freed and it goes to the twisting machine before it can finally become the bright and sparkling colorful lights we all love. But how do they actually work once they come out?

Here comes the scientific part. If you remember the flow of electric current, then you know electricity travels through a closed circuit to the filament of the bulb, causing it to light up. After traveling through the wire, the electricity finally makes it out of the bulb. But if the current was inclusive of only a single light bulb, our houses would only have one shining light rather than the abundance of variety and colors we have come to expect from the season. And where’s the Christmas spirit in that?

This is where the concept of series and parallel circuits come in. Simply put, the current in series circuits passes through a single line. However, they have one big flaw: when one light fails, all the lights go out. Meanwhile, parallel circuits’ current flows through multiple paths. Parallel circuits do not have this problem but it has its own share of flaws as well. For example, parallel connections cause the risk of overheating and fire hazards when it short circuits. Therefore, engineers decided to connect multiple series circuits in parallel. With this design, if one series of bulbs ends up failing, it will not affect the other series of bulbs because they are connected in parallel to the malfunctioning series. But what if one, singular bulb goes out in a series?

Luckily, Christmas lights have shunts which basically allow the electric current to continue flowing in an alternate path with lower resistance. Resistance is basically the measure of an object’s opposition to the flow of current. Therefore, lower resistance means an object resists the flow of current much less. Shunts are thus wrapped beneath the filament of holiday lights, becoming insulators which means that electricity cannot pass across them for as long as the filament remains functional. Normally, the current would avoid the shunt because it has higher resistance than the filament. But if the filament burns out, the heat from the burnout will cause the shunt’s coating to melt, allowing the electric current to travel through the less resistant path, aka the shunt. As such, even if one bulb burns out, the circuit will stay closed and the other lights will keep shining.

But when a short circuit happens under steady voltage, it creates a spike in current which could lead to a multitude of problems from the bulb burning out to actual fires. Thankfully, fuses are present in holiday lights. When the current increases to a hazardous level, the fuse overheats and it eventually melts. But don’t worry—the fuse safely opens the circuit and effectively prevents many disasters.

However, shunts are actually not used in all holiday lights. LED holiday lights have been emerging which work a bit differently than traditional Christmas lights. These use light-emitting diodes to produce lights instead of a filament. When traditional holiday lights fail, it causes an open circuit which causes the resistance to drastically increase too high for the current to pass across; something quite different happens when LED lights fail. Instead, they typically short circuit which creates a path of lesser resistance. As mentioned earlier, the role of a shunt is similarly to create a path of lower resistance so electricity can keep flowing. Since LED lights already short circuit, there is no need for shunts in LEDs. Similar to traditional bulbs, they are also wired in a series-parallel arrangement.

Christmas lights are truly one of the many things that complete Christmas. Beyond just brightening the interior of our homes, they also brighten our spirits and touch our hearts. While we only keep the Christmas lights up during the holiday season, the bright and merry feeling that Christmas lights give us should never go away. So when we begin taking our holiday lights down, let us be the light for others in our own unique ways. Just like how Edison’s invention of the light bulb sparked Johnson’s invention of Christmas lights, we can undeniably inspire others with our own light. The discovery of Christmas lights is proof that it only takes one light to light another. Moreover, as inventions from centuries ago like Christmas lights are constantly being reinvented, we must also keep growing and evolving to become the best versions of ourselves. As we continue to navigate the world and all its darkness, let us be the light brightening this darkness today for a brighter tomorrow.