There are two kinds of Christmas tree light. One kind, if one of the bulbs blows, the whole chain is dead, while for another kind, the chain works even if some of the lights are out. What's the difference, and why are the two methods both used?

Here are the two circuits:

The upper diagram is the series circuit, in which all the current goes through all of the light bulbs, so that if any one of them fails, the whole chain goes out. The lower diagram is a parallel circuit, in which the current has many independent paths. In the parallel circuit, each light bulb is connected to the two wires that lead to the power company, and so each sees 110 V. The light bulbs would be very low wattage, and designed for 110 V (which is very easy to do). In the series circuit, the light bulbs divide up the applied voltage -- if there are 18 bulbs in the string, each one is a 6.1 V light bulb. So perhaps these are light bulbs that were designed for a 6 V lantern battery.

I don't know why the series circuit was ever used. It uses half as much wire, and has the advantange that if we get a short circuit (as indicated by the red line, which represents a piece of aluminum foil tinsel that has managed to contact both wires)
the most likely outcome will be that one of the six bulbs to the left will blow out (they now are getting one-sixth of 110 V, which is more than they are designed for) and the line will become inactive -- the bulbs are acting like fuses; in contrast, a short in the parallel circuit will cause a very large current that might cause the tinsel to catch on fire (though the house circuit breaker will soon take the whole circuit out of service).

Is it safer for children to be playing with the series light bulbs? After all, they are only 6 Volts ... No, because when one of the bulbs is disconnected, now the entire 110 V appears across the nonfunctioning bulb (the other bulbs are conductors), and you can get just as bad a shock as you can from the parallel circuit.

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