Why is it that old Christmas tree lights were wired in serial so that if one bulb broke the whole string would fail? Surely parallel wiring has been around as long as serial so what was the advantage to doing it the old way?
making the bulb socket connections is easier. serial wiring is a single wire end to each terminal, two wires and two terminals. parallel wiring is two wire ends to each terminal, four wires and two terminals.
It was cheaper, used half the wire.
Current little incandescent Christmas lights are wired in series but the bulbs have shunts built into them so if one goes out, current still flows.
Well, it only uses half the wire if the lights are arranged in a loop. If they are a string, then it uses more-or-less the same amount.
IIRC, it wasn’t so much to save wire as it was due to the general difficulty of making tiny bulbs that could run directly on 110 volts and not explode back then.
It’s relatively quite easy to make bulbs that run safely on 2 volts, then to connect 55 of them in a series string.
Or it’s simply cheaper than making small bulbs with higher voltage filaments, rather than less difficult. Doing it this way means you don’t have to wipe out the savings by providing them with a step down transformer, although less convenient. Note that in the days of vacuum tube electronics, they would sometimes simply wire tube filaments and possibly a resistive load in series rather than have the added cost of a 5 volt filament winding on the power supply transformer.
This sounds like a reasonable answer.
From this link, it looks like some very early Christmas bulbs were rated at 15v.