Series or Parallel: Two Bulbs, Two Ways to Wire Them

Put two bulbs in one circuit and there are only two ways to do it: send the current down one path, or give it two paths. The first is a series circuit, the second a parallel circuit. This experiment settles the difference without any definitions — you unscrew one bulb and watch what the other one does.

Students can recite "the current is the same everywhere in a series circuit" and "branches in parallel have the same voltage across them" long before either sentence is useful to them. Usually that is because they have never wired the same two bulbs both ways and seen where the two circuits part company.

Safety first (read this before you start)

• Use dry cells only — 1.5 V each, two at most. Never run wiring experiments from a household wall outlet under any circumstances.
• Never join the two battery terminals with a bare wire. That is a short circuit: the wire and the battery heat up fast, hot enough to burn fingers or make the cell leak.
• Open the switch or lift out a cell before you change any connection. Rewiring a live circuit is how accidental shorts happen.
• Match the bulbs to the battery. A 2.5 V bulb takes two dry cells at most; three or more will burn out the filament.

Materials

• 2 dry cells and a battery holder (one cell works too, but two make the effect clearer);
• 2 small bulbs of the same rating — 2.5 V, 0.3 A is typical — and 2 lamp holders;
• 1 switch;
• 6–8 leads with crocodile clips;
• An ammeter (0–0.6 A range) if you have one. The experiment works without it — brightness and the unscrewing test are enough to reach the conclusion.

Procedure

1. Wire the series circuit first. From the positive terminal, go to the switch, on to L1, from L1 straight to L2, and back to the negative terminal. One loop, end to end, with no branch anywhere.
2. Close the switch and note how bright the two bulbs are. Compare that with a single bulb on the same battery.
3. Unscrew L2 (or lift it out of its holder) and see whether L1 stays lit. Put it back afterwards.
4. Now wire them in parallel. Open the switch. Run the positive terminal through the switch to a junction; from that junction split into two branches, one through L1 and one through L2; bring the two branches back together and return to the negative terminal.
5. Close the switch and note the brightness again, side by side with what you saw in series.
6. Unscrew L2 again and see what L1 does this time.
7. If you have an ammeter, put it in the main line, then in the L1 branch, then in the L2 branch, and record all three readings.

What you should see

With two dry cells and two 2.5 V bulbs, results come out roughly like this:

• In series: both bulbs light, both noticeably dimmer than a single bulb on the same battery, and both equally bright.
• In series, L2 unscrewed: L1 goes out immediately. The one path is broken and the current has nowhere to go.
• In parallel: both bulbs light, and both are much brighter than in series — close to a single bulb on its own.
• In parallel, L2 unscrewed: L1 carries on as before, with barely any change in brightness.

With an ammeter in the parallel circuit, the three readings line up like this. The numbers depend on your bulbs and battery; the relationship does not:

In the series circuit, the ammeter reads the same wherever you insert it. That is what "the current is the same everywhere" means, and students should verify it by moving the meter three times themselves rather than hearing it asserted once.

The science: one path or two

A series circuit connects the components one after another in a single loop with no branches. The current has only one route, so exactly the same current flows through every bulb:

I = I₁ = I₂

The supply voltage, meanwhile, gets shared out between the two bulbs, and the shares add up to the total (U = U₁ + U₂). Two identical bulbs take half each, which is why both are dimmer than a single bulb would be.

A parallel circuit connects both ends of each bulb to the same two points, so each bulb sits on its own branch. Every branch therefore has the full supply voltage across it:

U = U₁ = U₂,  I = I₁ + I₂

Each bulb gets the whole voltage, so each one lights properly; the main line carries both branch currents together, which is why the battery runs down faster.

The unscrewing step is really a test of paths. A series circuit has one route, so breaking it anywhere kills the whole thing; a parallel circuit has two, so losing one leaves the other untouched. This is not something to memorise — it follows: count the paths first, and every other conclusion falls out of that.

Series and parallel around the house

• Household lights are in parallel. Switching off the living room does nothing to the bedroom, and every fitting gets the full mains voltage. Wired in series, one switch would darken the house and no lamp would be bright enough anyway.
• A switch is in series with the light it controls. It has to sit on that path, because opening it must break that path.
• Old Christmas light strings were wired in series, which is why one dead bulb killed the whole string and you had to test them one at a time. Newer strings are parallel or split into short series sections, precisely to avoid this.
• Cells inside a battery holder are in series, so their voltages add: two 1.5 V cells give 3 V.
• Stairwell emergency lights and street lights are parallel — one failure leaves the rest running, and a single branch can be isolated for maintenance.

Five things students get wrong

• "Parallel bulbs are brighter, so parallel wastes more power." The first half is right and the second half has the causation backwards. Parallel bulbs are brighter because each gets the full voltage and draws more current, which means more power — the extra power consumption is the consequence of the brightness, not its cause. The battery really will run down faster.
• Calling it parallel because the wires branch. What matters is not the shape of the wiring but whether both ends of each component connect to the same two points. Some layouts loop wire all over the place and still offer only one route. There is one reliable test: start at the positive terminal and trace with a finger, looking for a genuine fork.
• "In series, the bulb nearer the positive terminal is brighter." It is not. The current is the same everywhere, so two identical bulbs are equally bright. The intuition comes from imagining current as water that the first bulb "uses up". Current is not consumed; energy is.
• Connecting an ammeter across a bulb. An ammeter goes in series with whatever it measures, because its resistance is nearly zero. Put it across a bulb and you have handed the current a near-resistance-free shortcut — a short circuit, with the needle slamming over and possibly a ruined meter. It is the voltmeter that goes across two points.
• "Two bulbs in parallel are each a bit dimmer than one alone." In theory each keeps its full voltage and its brightness. In practice they do dim very slightly, because a real dry cell has internal resistance and drops more voltage inside itself as the total current rises. Worth mentioning to students — but don't let it overshadow the main result.

Teaching notes

• Time and grouping: 35 minutes, pairs. Per set: battery holder, two bulbs and holders, a switch, eight leads.
• Say this before they start: never bridge the battery terminals with a wire, and always open the switch before rewiring.
• Prep that pays off: use bulbs from one batch. Mismatched ratings blur the key observation that the two are equally bright.

The step most often skipped. Unscrewing a bulb tends to get treated as a bit of fun and dropped for time, when it is the strongest evidence in the lesson — stronger than the brightness comparison. Brightness is continuous and arguable; lit or not lit is binary and settles the matter. Do it in both circuits and let students state the result themselves.

How to tell they have it. Give them a circuit diagram they have not seen and ask which components are in series and which are in parallel, and why. The reason should be "the current does or does not fork here", not "it looks like the one in the book". Then ask what happens to the other bulb if this one is removed. A correct answer means the idea of paths has landed.

Take it further

• In the simulator, replace the matched pair with one bright and one dim bulb and run both circuits again. In series the dimmer bulb takes the larger share of the voltage, which almost nobody predicts.
• Try three bulbs. How much dimmer is a series string of three? How long does the battery last with three in parallel? The second one is worth actually timing.
• Play "find the break". Have a partner quietly loosen one connection in a series circuit while you look away. Using a single lead, bridge across one section at a time; when the bulb lights, the fault is in the section you just bypassed. This is exactly how an electrician traces an open circuit.
• Think about a traffic light. Are the red, amber and green lamps in series or parallel, and why? (Parallel — each has to be switchable on its own, and one failure must not take the others down.)
• Move the switch from before the bulb to after it and close it again. Does the bulb still light? This small change shows that the order of components in a series loop makes no difference, as long as they are all on the same path.

When a student's first move on an unfamiliar circuit diagram is to trace the current with a finger and count the paths, this experiment has done its job.