Necessary conditions for using test series lamp for testing.
Answers
A series test lamp is a way that electricians would test or find a “short circuit” somewhere in a residential building. Back when homes used “screw in fuses,” that had the same thread pattern as a light bulb (known as an Edison Thread), if one circuit in a house was repeatedly blowing fuses, the electrician would remove all loads from that circuit by unplugging all appliances and turning off all of the room switches in that circuit, remove the blown fuse and replace it with a regular high wattage light bulb rated for the house’s voltage (117V in USA). If the lamp glowed at all, it meant that there was load (less than infinite resistance) in the house wiring. If it did not light up at all, the electrician would go around turning switches on in that circuit to see which “switched branch of the circuit” had a partial short (60 ohms) or a dead short (0 ohms). If the series test lamp burned very dimly, it meant that the switched circuit had a small enough load that it would not blow the fuse. After checking the switched branches of the circuit, he would start plugging in appliances commonly used on that circuit. Whichever appliance caused his light in the fuse socket to burn brightly was the offending appliance that had a short in it.