1 year ago
Last edited at 2:27AM on 5/30/2012
Sounds like a series-parallel circuit to me. Some of the bulbs are wired in series while others are wired in parallel. There are apparently three sets of 3 bulbs each wired in series, but the three sets are wired in parallel with each other. If any one bulb burns out, the other 2 bulbs in that series will not glow because there is no longer any electricity flowing through that part of the circuit.