Why don't your Christmas lights work? Sadly, in the event of a string of lights going out on your tree, replacing the strand is usually the best option. It's pretty difficult to find that one bulb that's causing the problem. However, this is a great time to look at the difference between series and parallel circuits.

A Simple Circuit

Just about the simplest circuit you can create uses just one battery and one light bulb. In one fun activity, you can give a person one wire, one battery and one light bulb. Ask the person to make the bulb shine. It's possible to get it to work, but it can still cause some trouble for people.

In order to get the bulb to glow, you need a complete circuit. There has to be a path for the current to come out of one end of the battery, go through the filament in the bulb and then return to the battery. Here's how that could work.

A simple circuit with a battery and bulb. Rhett Allain

When current passes through the filament, it makes the filament get super hot. Super hot things glow. That's all the bulb does. But why the glass on the outside? That is there to keep air away from the super hot filament. With air in contact with the filament, it would burn out.

If you take away any one of the parts for the complete circuit, there would be no current and no glowing bulb.

Light Bulbs in Series

Now you want to connect two light bulbs to battery. Here is one way you could do that.

A circuit with two lightbulbs connected in series. Rhett Allain

In this case, there is just one path for the current to follow. The current that goes through the first bulb also goest through the second bulb. If you remove any part of this circuit (like take out a bulb), everything goes out. If you wanted, you can make a similar circuit with 10 or 20 bulbs. Just connect them all in a line so that the same current goes through each one. This is a series connection.

Light Bulbs in Parallel

There is another way to connect two light bulbs. Check it out.

A circuit with two lightbulbs in parallel. Rhett Allain

The difference in this circuit is that each lightbulb has its own "path" to the battery. If you break one of these bulbs, it doesn't really do anything to the other bulb.

Here is a quick demo showing the difference between bulbs in series and bulbs in parallel.

Oh, and just like the lightbulbs in series you can do this with many, many bulbs.

Christmas Lights

Now we can get to the fun stuff. What about the lights you put on your Christmas tree? Are those lightbulbs in series or parallel? What do you think? Here are two hints.

Hint number 1: if you pull out one of the lightbulbs from a strand, the rest of the lights go out. Ok, this isn't always true. Many lights are actually two sets of strands. If you pull out a lightbulb, half the strand goes out.

Rhett Allain

Here you can see that this strand has one bulb removed and non of the other lights are on (even though the power is on).

Hint number 2: In some cases, one of the lightbulbs in a strand may "burn out". When a lightbulb burns out, typically it will be due to a broken (or melted filament). In this case, the strand would look like this.

Rhett Allain

Notice the one bulb is indeed out but the other bulbs are on. But is the filament actually broken? Yes, if you look closely, you can see something like this.

A broken filament in a Christmas light. Rhett Allain

Ok, now what's the answer? Are the bulbs in series or parallel? Based on hint 1, it seems the lights are in series. Based on hint 2, it seems they are in parallel. The answer is that the lights are in series. Yup.

But how can this be? How can you have one light with a broken filament and still have the other lights on? In the physics world, we call this "the magic of Christmas". Ok, not really. The answer is that the lights are connected in series but the bulbs have a trick.

Let's take a closer look at one of the light bulbs in the strand.

A shunt wire (bypass wire) in a Christmas light. Rhett Allain

This is a view of a broken bulb. At the end of the metal posts you can see the broken filament - so this is a burned out bulb (plus I broke the glass so you could see better). But what about that metal wire wrapped around the posts (indicated with the red arrow)? What is that for? If it was just a normal wire, this would short out the bulb. It would mean that most of the current going through this bulb would run through this wire and not through the filament. With very little current, the filament would not glow.

So, in order for this bulb to function properly in a strand (with a good filament), there shouldn't be a current going through this bypass wire. Ok, now let me make some guesses as to how this magic Christmas light works. Yes, I could ues the internet to find out more about this bulb, but I really like guessing.

Like I said, in a functioning bulb this wire does not conduct electrical current. There must be some type of insulator between the bypass wire and the filament post.

When the filament burns out, all the current going through all the bulbs in series stops.

Without current going through the bulbs, there is a high potential difference between the bypass wire and one of the filament posts (I will show why below). This high voltage (120 volts) across a very short distance produces a very high electric field.

If the electric field is high enough, it could cause and electric breakdown in the material and turn it from an insulator into a conductor.

Boom. The rest of the lights work now (but it probably doesn't make a boom sound).

But how does a broken filament make a high voltage right at the bulb that is broken? Let's pretend like we have a different circuit. In this one, I have 5 lightbulbs in series connected to a battery. Oh, and all the lightbulbs work.

Resistors in series Rhett Allain

Suppose each lightbulb has a resistance R (it doesn't matter than lightbulbs are non-ohmic). If I have a 10 volt battery connected to them, then the total change in electric potential around the loop must be zero volts. Also, since the bulbs are in series, they have to have the same electric current (I). This means that I can write:

So each bulb has a 2 volt electric potential across it. But now let's say one of those bulbs break. With an open circuit, there is no longer a current. Here is the new diagram.

Resistors in series with a break in the circuit. Rhett Allain

Even though there is a break, the change in electric potential around this loop still has to be zero volts. But since there is no electric current, the change in potential across each resistor is zero. This means that the full 10 volts must be across the gap. This 10 volt gap potential creates a large electric field that breaks down the insulator. That's the magic of the Christmas lights.

My Lights Still Don't Work.

These lightbulbs have this awesome bypass wire inside the bulbs. However, if the light bulb is not even plugged in even the bypass wire won't work. So, if you have a strand that doesn't work here are some things you can try.

1. Is it plugged in? Yes, you should check this. A strand of lights that isn't plugged in won't work. If you have lights plugged in outside and they get wet, they will likely trip your Ground Fault Interrupter (GFI). You have these on the outlets in your bathroom (with the test and reset button). When the circuit gets wet it can trip this.

2. Check the fuse. I'll assume that you checked your household circuit breaker. If that was tripped, you could probably tell because of the other things that weren't working. But there are also fuses in the little plug on the end of the strand. There is a sliding door and it looks like this.

Rhett Allain Fuses in a strand of Christmas lights.

If you connect too many strands of lights together then the current will be too large. The fuse will go out to prevent these things from getting too hot. However, sometimes these little fuses just get loose after storing the lights in the off season. It doesn't hurt to check them.

3. Look for a loose bulb. If a strand doesn't work and you checked the other stuff, there's a good chance the problem is a loose bulb. It can be quite a challenge to locate a loose bulb. Normally, I just push on each bulb to see if I can get the strand of lights to come on. There is also a special tool you can use. Mine looks like this:

A Christmas light detector. Rhett Allain

I'm not completely certain, but I think this thing detects small electric currents. If you have a loose bulb, there will be a slight alternating current going to and from the outlet to the break. On the other side of the break there won't be the same current. You can detect a current by detecting the magnetic fields that go along with the current. But now I'm not so sure this is the way these detectors work. I'll have to think about this is a little bit.

4. Check for squirrels. Yes. Look at these lights on a tree in my front yard.

A broken strand of Christmas lights. Rhett Allain

I guess squirrels hate Christmas.

Oh, and just for fun I made a video version of this same thing.