There are two kinds of Christmas tree light. One kind, if one of the bulbs blows, the whole chain is dead, while for another kind, the chain works even if some of the lights are out. What's the difference, and why are the two methods both used?

Here are the two circuits:
The upper diagram is the series circuit, in which all the current goes through all of the light bulbs, so that if any one of them burns out and becomes a nonconductor, the whole chain goes out. The lower diagram is a parallel circuit, in which the current has many independent paths.

In the parallel circuit, each light bulb is connected to the two wires that lead to the power company, and so each sees 120 V. The light bulbs would be very low wattage, and designed for 120 V (which is very easy to do). In the series circuit, the light bulbs divide up the applied voltage -- if there were 20 bulbs in the string (as in the picture), each one would get 6 V across it, and so we would use a light bulb designed for 6 V.

The series circuit is awkward, because when the chain goes out it is hard to find what has gone wrong. Its main advantage is that it uses half as much wire.

It might seem that there is a safety advantage in using 6 V light bulbs. Indeed, there would be a good safety reason for using a 6 V circuit (with 6 V light bulbs connected in parallel). However, this is an illusion: there is still 120 V across the whole chain, and large voltages between different parts. Furthermore, consider the case that one of the bulbs has burned out, and we are trying to find it: now there is no current in the circuit, and then no voltage drop across the good light bulbs. The entire 120 V is across the nonfunctioning bulb, and if you get your finger into the circuit trying to replace the dead bulb, you will get a nice shock.

There is a difference between the two kinds of circuits, that appears if we get a short circuit (as indicated by the red line, which represents a piece of aluminum foil tinsel that has managed to contact both wires). The most likely outcome in a series circuit will be that one of the six bulbs to the left will blow out (they now are getting one-sixth of 120 V, which is more than they are designed for) and the line will become inactive -- the bulbs are acting like fuses; in contrast, a short in the parallel circuit will cause a very large current that might cause the tinsel to catch on fire (however, the house circuit breaker will soon take the whole circuit out of service).

Why study circuits?
The section on circuits