Many years ago holiday lights used large bulbs designed to work on 120 volts. Those strings were wired in parallel. Today holiday lights are wired in series so that the tiny bulbs have only low voltages across them. If a lamp burns out because the filament fails, there will be no current through the string. But, there will be 120 volts across the failed bulb. The bulb has a wire touching the two thick wires that deliver current to the filament. The wire is covered with a thin insulating film. The film remains insulating when the voltage difference across the bulb is small, but when it becomes 120 volts, sparks break the insulating film and weld the wire to the thick wires. This short-circuits the bulb, completing the circuit through the remaining lamps.

A series circuit consists of electrical devices such as resistors, batteries, and switches arranged in a single line. There is only one path for the charges to flow through, and if there is a break anywhere in the circuit, the current will drop to zero.

A parallel circuit allows the charges to flow through different branches. For example, the wire from the battery would be connected to one terminal of each of three bulbs. The other terminals are connected together and to the negative terminal of the battery. The charges now have three separate paths through which they can flow. If one bulb burns out or is removed from the socket, that bulb would no longer light, but the current through the other two lamps would not change. They would continue to glow.