Series and Parallel Circuit Experiment: Why Bulbs Dim

Unscrew one bulb and the other one goes out too. That is the moment a series and parallel circuit experiment earns its keep, because it is the thing every child has encountered without understanding: the Christmas string where one dead bulb kills the whole row. Rewire the same two bulbs the other way and unscrewing one leaves the other burning happily. Two arrangements, two completely different behaviours, from identical parts. There is one catch, and it is worth knowing before you shop: LEDs cannot show you this, and the article explains why.

Key Takeaways

• You’ll wire two 1.5-volt bulbs in series and then in parallel on one cell, compare brightness, then unscrew one.
• It teaches that in series the current is shared and the voltage splits, while in parallel the voltage is shared and the currents add.
• Best from about age 8 for the build, from 11 for the current measurements.
• What usually goes wrong: using LEDs. They are nearly constant brightness above their threshold and then abruptly off, so the dimming never appears.
• One 1.5-volt cell powers everything here. Never a wall socket or a mains adapter.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Miniature incandescent bulbs, 1.5 V	3, E10 screw base	Hardware store, as torch bulbs	Bulbs from an old Christmas string. LEDs will not show the effect
Bulb holders, E10	3	Electronics or hardware shop	Tape the bulb base to a coin and the side to a stripped wire
AA or D cell with holder	One 1.5 V cell	Supermarket, electronics shop	Do not substitute a coin cell (see the Don’t row)
Alligator-clip leads	6	Electronics shop	Solid wire with 1 cm stripped at each end
Digital multimeter	One	Hardware store	None for the measuring half of the activity
Paper and pencil	For the circuit diagrams	Drawer	—

Time: prep 10 min, experiment 40 min, cleanup 5 min Cost: around 10 dollars for bulbs, holders and leads, reusable for years Mess level: low

Do the Experiment: A Series and Parallel Circuit Experiment

Step 1 — Build one bulb on one cell and call it the reference

Cell, lead, bulb, lead, back to the cell. Note how bright it is, and take a photo with locked exposure so you have something to compare against later. Everything after this is judged against this single-bulb brightness.

Step 2 — Predict both circuits before building either

Draw two diagrams: two bulbs in a single loop, and two bulbs each with their own loop back to the cell. Ask, for each one: brighter, dimmer or the same as one bulb? Then ask what happens if you unscrew one. Write all four predictions down. Most people get at least one wrong.

Step 3 — Wire the two bulbs in series

One loop: cell, bulb one, bulb two, back to the cell.

What you should see: both bulbs light, both noticeably dimmer than the reference, and both equally dim. The “both equally” part matters, because many children expect the first bulb to be brighter.

Step 4 — Unscrew one bulb in the series circuit

What you should see: the other goes out instantly. There is only one path, and breaking it anywhere breaks it everywhere.

Step 5 — Rewire the two bulbs in parallel and repeat

Each bulb gets its own pair of leads back to the cell’s two terminals.

What you should see: both bulbs at close to the reference brightness. Unscrew one and the other keeps glowing, barely changed. The battery, however, is now working twice as hard, and if you leave it running you will notice it fading sooner.

Step 6 — Put the multimeter in the loop and measure current in three places

Set the meter to DC amps in series with the circuit. Measure the series circuit’s current at three different points in the loop: between the cell and bulb one, between the two bulbs, and between bulb two and the cell.

What you should see: the same reading at all three points, within the meter’s precision. Then measure the parallel circuit’s total current and each branch separately. Total current roughly doubles compared with a single bulb, and each branch carries about what a single bulb carried. Nothing is being used up anywhere.

What’s Actually Going On

A circuit is a loop, and charge does not accumulate or disappear in it. In a single loop, every electron that leaves the cell must pass through every component, so the current is identical at every point. That is why your three measurements in step 6 agree, and it is the direct refutation of the most common idea children hold about electricity: that current is a substance that gets spent as it travels round, so the first bulb takes some and leaves less for the second. McDermott and Shaffer documented exactly this belief surviving formal instruction in university physics students, and showed that it has to be confronted with measurements rather than corrected verbally (McDermott & Shaffer, 1992; Shaffer & McDermott, 1992).

What does get divided in series is voltage. The cell provides about 1.5 volts across the whole loop, and that has to be shared between the two bulbs, so each gets roughly 0.75 volts. Less voltage across a bulb means less current through it and much less power dissipated in the filament, so it runs cooler and dimmer. Resistances in series add, which is why the total current falls (HyperPhysics on resistors in series).

Parallel is the mirror image. Both bulbs are connected directly across the cell, so both get the full 1.5 volts and both light normally. The two branches each draw their own current and those currents add, so the cell delivers roughly twice as much as before. Total resistance goes down rather than up (HyperPhysics on resistors in parallel). House wiring is parallel for exactly this reason: every socket gets full voltage and switching off a lamp does not darken the kitchen. The Physics Classroom’s treatment of the two connection types is a good place to send an older child who wants more.

Two honest complications. First, the numbers will not obey simple arithmetic. A filament’s resistance rises strongly with temperature, so a dim, cool bulb has markedly lower resistance than a bright, hot one. Put two 1.5-volt bulbs in series and the current will be more than half the single-bulb value rather than exactly half, because both filaments cooled down and got less resistive. This is not experimental error. Incandescent bulbs are nonlinear components, and noticing that from your own data is a better lesson than a tidy agreement with Ohm’s law would be.

Second, the reason this experiment needs real filament bulbs. An LED conducts almost nothing below its forward voltage and then rises very steeply, so with two in series on 1.5 volts you get darkness, and with enough voltage you get near-full brightness. There is barely a dim region to observe. An incandescent filament, by contrast, glows smoothly across a wide range of voltages, which is precisely what makes it a good teaching component and precisely why it is disappearing from shops.

How to Explain It to Your Kid

Ages 5–8: One road or two roads

Electricity has to go all the way round in a loop and come back. If both bulbs are on the same road, everything that goes through one has to go through the other, and they have to share the push from the battery, so both end up dim. If each bulb has its own road, each one gets the whole push and both are bright. Let them do the unscrewing themselves in both versions. The difference between “they both go out” and “the other one stays on” is the whole idea, and they will remember doing it.

Ages 9–12: Voltage splits, current doesn’t

Give them the two rules as a pair. In series, the same current goes through everything and the voltage divides up. In parallel, everything gets the same voltage and the currents add. Then have them test the first rule with the meter at three points in the loop, because that is the measurement that kills the “it gets used up” idea for good. Our guide to voltage, current and resistance with the water analogy gives them a picture to hang the two rules on.

Ages 13+: Nonlinearity, and a graph that misbehaves

Have them predict the series current using Ohm’s law and the single-bulb resistance they measured, then measure it and find the prediction wrong in a specific direction. Working out that cooler filaments have lower resistance, and therefore that the measured current should exceed the prediction, is a genuinely good piece of reasoning. Then plot current against voltage for one bulb by adding cells one at a time. The line curves. Ohm’s law is a property of some components, not a law of nature.

The question to ask: “If I put three bulbs in series instead of two, will each one be two-thirds as bright as before, or less than that?”

Why Teaching This Early Pays Off

Series and parallel is the first place where children meet the idea that the arrangement of components matters as much as the components themselves. Same two bulbs, same cell, radically different behaviour. That is the beginning of systems thinking, and it transfers: pipes, roads, networks, supply chains and organisations all behave differently depending on whether things are in series or in parallel.

The misconception it kills is unusually durable. Believing current gets consumed is not a silly mistake; it is a sensible inference from everyday experience with fuel and batteries running down. The research literature treats such ideas as productive raw material that has to be engaged rather than overwritten, and the three-point current measurement is engagement rather than contradiction. It is also worth being honest about scale: one afternoon with two bulbs will not inoculate anyone against the misconception for life, and it is likely to reappear in a new form next year. Having a remembered measurement to point back to is what makes the second conversation shorter than the first.

Before You Start: What Parents Should Know

Best age	8 for the building and unscrewing; 11 and up for the multimeter work and the nonlinearity
Supervision	Adult sets up the meter and supervises connections. Children wire, unscrew and record
Real hazards	Bulbs get hot: the glass of a 1.5-volt bulb becomes warm and the filament inside is at thousands of degrees, so let it cool before unscrewing. A cell shorted directly with a lead gets hot fast, so keep one bulb in every loop. Broken glass if a bulb is dropped
Don’t	Don’t substitute a coin cell for the AA. It is tempting because they are flat and cheap, but a swallowed button cell can burn through a child’s throat or oesophagus in as little as two hours, according to the US Consumer Product Safety Commission. If a project genuinely needs one, it stays taped into a closed holder and is counted in and out. And never, for any reason, power a home-built circuit from a wall socket or mains adapter

When It Doesn’t Work

• Nothing lights at all. Nine times out of ten it is a contact rather than a concept. Check that the bulb is screwed fully home, that the clip leads grip bare metal and not insulation, and that the cell is the right way round in its holder. Then test each lead with the meter’s continuity setting.
• The series bulbs do not look dimmer. Check your bulb ratings. Two 1.5-volt bulbs on a 3-volt pack in series each get their full rated voltage and look normal, which is a correct result but not the one you wanted. Use a single 1.5-volt cell with 1.5-volt bulbs.
• One series bulb is brighter than the other. They are not identical bulbs. Check the ratings printed on the base or the glass. Mismatched bulbs in series genuinely do glow unequally, because the one with more resistance takes more of the voltage, and that is a bonus result if you explain it.
• The parallel circuit flattens the cell in minutes. Expected. Twice the current means half the life. Disconnect between tests, and mention that this is why a torch with more lamps needs bigger batteries.
• You can only find LEDs. Then switch from judging brightness to measuring voltage. Put a 100-ohm resistor in series with each LED, build both circuits, and measure the voltage across each LED and resistor. The voltage division is clearly visible on a meter even when the brightness difference is not.

Take It Further

Three, four, five in series: add bulbs one at a time, photographing with locked exposure and measuring current each time. Plot current against number of bulbs. The curve is not the straight line a naive calculation predicts, and the gap is the temperature effect on filament resistance.

Mixed circuits: build two bulbs in parallel with a third in series feeding them. Predict which is brightest before switching on, then measure voltages across all three. This is the first genuinely non-obvious circuit, and getting the prediction right requires both rules at once rather than either one alone.

Where This Series and Parallel Circuit Experiment Fits in a Sequence

A child needs one complete working circuit under their belt first, so an introduction to circuit literacy is the step before this if the words “loop” and “terminal” are still new. Afterwards, the natural pair is how resistors work, because the series and parallel rules are much easier to test on resistors than on bulbs once the ideas are in place. The nonlinearity thread leads to why wires get hot and what a fuse actually does, which is the same physics of resistance and heat applied to the safety device in your house.

What to Watch For Over the Next 3 Months

• Week 4: they explain the Christmas-light problem to somebody else, correctly, using the word series.
• Month 2: they ask why the kitchen lights do not dim when the kettle goes on, which is a parallel-wiring question and a good one.
• Month 3: they stop saying electricity gets used up and start saying energy does, which is the correct distinction and a real step forward.

Frequently Asked Questions

Why do both bulbs go out when I unscrew one in series?

Because a series circuit has exactly one path. Current has to pass through every component in turn, so removing any one of them breaks the loop everywhere. In a parallel circuit each branch has its own path, so the others carry on unaffected.

Can I use LEDs instead of bulbs?

Not for the brightness comparison. An LED conducts almost nothing below its forward voltage and then rises very steeply, so you see either darkness or near-full brightness with little in between. If LEDs are all you have, add a resistor in series with each one and measure voltages with a meter instead of judging by eye.

Why doesn’t the current halve exactly when I add a second bulb in series?

Because a filament’s resistance rises with its temperature. Two dim bulbs run cooler than one bright one, so each has lower resistance than you measured on the bright one, and the current comes out higher than a simple calculation predicts. Incandescent bulbs are nonlinear.

Is house wiring series or parallel?

Parallel. Every socket and lamp is connected across the full supply voltage, so each gets what it needs and switching one off does not affect the others. A series house would mean every appliance dimming every other one, and one failure taking down the lot.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. McDermott, L. C., & Shaffer, P. S. (1992). “Research as a guide for curriculum development: An example from introductory electricity. Part I: Investigation of student understanding.” American Journal of Physics, 60(11), 994–1003. https://doi.org/10.1119/1.17003
2. Shaffer, P. S., & McDermott, L. C. (1992). “Research as a guide for curriculum development: An example from introductory electricity. Part II: Design of instructional strategies.” American Journal of Physics, 60(11), 1003–1013. https://doi.org/10.1119/1.16979
3. Nave, R. “Resistors in Series.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/serres.html
4. Nave, R. “Resistors in Parallel.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/parres.html
5. The Physics Classroom. “Two Types of Connections.” https://www.physicsclassroom.com/class/circuits/Lesson-4/Two-Types-of-Connections
6. U.S. Consumer Product Safety Commission. “Button Cell and Coin Batteries.” https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Button-Cell-Coin-Battery-Information-Center