Stop the current from flowing through one resistor.
- If all of the resistors fail at that point, it is a series circuit.
- If all of the other resistors remain on, it is a parallel circuit.
- If some of the other resistors remain on, but some fail, it is a combination circuit.
This is a common problem with Christmas lights. The strings made of a series circuit (usually 2 wires) are the frustrating ones; if one bulb goes out, they all go out... and it's painful to try to locate the offender. The parallel circuit light strings (usually 3 wires) are much less frustrating, because only a single bulb goes out. Some of the new "icicle" lights are combination circuits, so you may have a whole icicle go out, but the rest of the strand works.