If you're looking to sell an antique incandescent lightbulb, you'll get a much higher price if the bulb is working, but don't screw it into a contemporary light fixture. Testing it for continuity, using a multimeter, is a more reliable method, and it won't burn out the bulb. Continuity testing is also a reliable way to troubleshoot pesky string lights that have a habit of burning out the day before Christmas.
Testing a Lightbulb
You can use a dedicated continuity tester, which consists of a LED attached to a pair of wire leads, or a multimeter. In either case, the device passes a small current through the bulb and detects whether the circuit is open or closed by measuring resistance. If the circuit is open -- which means the bulb is bad -- resistance is infinite. Resistance is zero if the bulb is good.
Set the multimeter to measure resistance in ohms. If there is more than one resistance setting, choose the most sensitive one.
Put the leads of the multimeter or continuity tester together. The multimeter should read 0 when you do this, and the continuity tester should light up. If that doesn't happen, clean the leads with alcohol.
Touch one lead to the small metal knob on the bottom of the bulb and the other lead to the bulb threads. If the bulb doesn't have threads, put the second lead on one of the two knobs that protrude from the side of the base. The bulb is good if the meter reads zero of the tester bulb lights up.
When testing continuity, polarity isn't important -- the leads are interchangeable.
Testing String Lights
You can use a continuity tester to test an entire string of lights as well as to locate a burnt-out bulb in a loop wired in series. Typically when a bulb burns out in such a loop, all the bulbs in the loop go out.
Unplug the string from the wall socket. Insert one lead into one of the sockets on the female end and place the other lead on one of the male prongs on the other end of the string. If the reading isn't zero or the bulb doesn't light, switch one of the leads to another prong or socket. Keep rearranging leads until you get continuity. If you don't get continuity, it's because the main string wire is broken. Look for signs of a break, such as damaged insulation and exposed wire.
Pry open the male plug, using a flat-head screwdriver, and remove the fuse. Test the fuse by putting a lead on either end. If you don't get a zero reading on the meter or the light tester light doesn't illuminate, replace the fuse. Your light string may have come with a spare fuse.
Locate a burnt-out bulb by removing each bulb carefully from its socket, beginning at one end of the string, and placing each lead on one of the wires on the bottom of the bulb. For larger sets wired in parallel, you may be able to see the single bulb that doesn't light when you plug in the set, or it may be part of a smaller loop upon which you can focus your testing.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.