There are two ways to wire up a row of bulbs. In series, they all sit on one single loop, like beads on a string. The same current has to crawl through every bulb, so each one is dimmer, and if you take just one out you break the only loop and they all go dark at once. In parallel, each bulb gets its own private loop back to the battery, so every bulb gets the full push and glows bright. Pull one out and the others don't even notice. That's why your house is wired in parallel — switch off a lamp and the rest of the lights stay on. Flip between series and parallel in the simulator and try removing a bulb in each.
Most people think wiring bulbs together always dims them or that removing one dims the rest. In fact it depends entirely on the wiring: series shares one loop and goes all-or-nothing, while parallel branches each get full voltage independently.
What's actually happening
The honest test of whether you understand a circuit is to ask what happens when you remove one bulb. Most people guess it dims the others, or brightens them, and the answer is: it completely depends on how they're wired, and the two cases couldn't be more opposite. Get this one distinction and a huge amount of electrical intuition falls into place.
In a series circuit, everything sits on a single loop. There is exactly one path for the current, so the same current passes through every bulb in turn — and the battery's push, the voltage, has to be shared out among them. Three identical bulbs in series each get a third of the voltage, so each one runs dim, and the more you add the dimmer they all get. Worse, that single path is fragile: unscrew any one bulb and you've opened the only loop there is, so every bulb goes dark together. That was the misery of old fairy lights. A parallel circuit flips all of this. Each bulb gets its own loop straight back to the battery, so each one sees the full voltage and burns at full brightness no matter how many you add. The current splits at the junction, sending a separate stream down each branch, and those streams merge again on the way back.
The consequence is the reason your home is wired the way it is. Every socket and light in your house is a parallel branch across the same mains voltage, so your kettle gets the full 230 (or 120) volts whether or not the lamp beside it is on, and switching one thing off leaves everything else running. The price you pay is current: parallel branches each draw their own, and all that current adds up in the wires feeding them, which is why a circuit can be overloaded and why the fuse or breaker is there to trip before the wiring overheats. Series still has its uses (a string of LEDs, a fuse in line with a device, sensors in a chain) but for anything you want to control independently, parallel wins.
Series shares one fragile loop; parallel gives each device its own full-voltage path, which is exactly why your house is wired in parallel.
- 1With a battery and two identical bulbs, wire them in a single loop (series): both light up, but noticeably dimmer than one bulb alone would.
- 2Unscrew one bulb. The other goes out too, because you've broken the only path the current had.
- 3Now rewire so each bulb has its own loop to the battery (parallel): both glow at full brightness, and unscrewing one leaves the other shining. That's your house.
Common questions
Every socket and light is a parallel branch across the same mains voltage, so each device gets the full voltage and switching one off leaves the rest running. Series wiring would mean one failure killed everything at once.
In series, removing one bulb opens the only loop, so every bulb goes dark together, the old fairy-light problem. In parallel, each bulb has its own loop, so the others carry on shining unaffected.
Each parallel branch draws its own current, and all of it adds up in the shared wires feeding them. Plug in too much and the total climbs until the breaker trips, stopping the wiring from overheating.