funny and interesting at the same time.
That's the part I was curious about....The shunt wire contains a coating that gives it fairly high resistance until the filament fails. At that point, heat caused by current flowing through the shunt burns off the coating and reduces the shunt's resistance...
Most of the time, those shunts don't work, as intended. The shunt has some kind of a dielectric coating that doesn't always break down at full line voltage.
Retiredspark, isn't the tester they sell just a little tic-tester? Seems like on old series strings without the shunt, you'd just be looking for the last place you had voltage. Not sure how it would work with a shunt, I guess it doesn't?...It consists merely of a piece of OXIDIZED aluminum wire, wrapped around the lead-in wires, just above the bead in the lamp. At normal operating voltage the oxide coating acts as an insulator, and the current goes through the filament. But when a lamp burns out, There is an OPEN CIRCUIT, and, in all series wiring, that puts the FULL LINE VOLTAGE across the defective lamp, and the 120 volts will "BURN" through the extremely thin oxide coating on the shunt, causing the shunt to actually short the lamp out....
...This very same principal was applied to the old series incandescent street lights. The open circuit volts here though was typically either 2400 volts, or 4160 volts, and the shunt was in the socket, between two prongs. It was called a "FILM-DISC CUTOUT" and was a pair of metal discs about the size of a dime, or a little smaller, separated by a thin piece of silk cloth. When a lamp burned out, the high voltage would puncture through the silk, shorting the lamp socket out, and completeing the circuit. A voltage/current regulator compensated for blown lamps, so the others would not get more voltage! The mini-lights have no regulator, so as they fail and short, the remaining ones get more voltage.