What a dimmer does
A lamp is brighter when more current flows through it. A dimmer controls the current. It is a variable resistor (also called a rheostat) joined in series with the lamp. The current flows through both, one after the other.
The current is I = V ÷ (Rlamp + Rrod). If Rrod grows, I falls, and the lamp gets dimmer.
The build (use batteries only)
You need: 2 AA batteries in a holder (3 V) or 4 AA (6 V), a small torch bulb with holder, a long soft pencil lead (2B or darker) or a pencil cut in half, crocodile clip leads, and a switch (or just a loose wire).
- Join the battery, the bulb and one end of the pencil lead in a loop.
- Use a second clip as the sliding contact. Clip it on the lead and join it back to the battery.
- Move the clip along the lead. Watch the bulb change.
- Add a switch in the loop so you can turn the bulb off.
Safety: use only batteries. Never use a wall socket for a project like this.
Why length and thickness matter
Graphite is a poor conductor compared with copper. The longer the path, the more the charges are slowed. R = ρ × L ÷ A: resistance grows with length L and falls with area A (thickness).
- Slide the clip away from the first end: L grows, R grows, lamp dims.
- Use a thicker lead: A grows, R falls, lamp is brighter at the same position.
Choose the rod to suit the lamp. If the rod is too short or thick, the dimming is tiny. If it is very long, the lamp may be very dim even at the start.
Design choices and honest limits
With a series rod, the lamp never goes completely off, because the rod always has a limited resistance. A switch is needed for that. Also, a series dimmer wastes energy: the electrical energy not used by the lamp turns into heat in the rod. The rod can get warm. Real home dimmers use electronic parts that switch the current on and off very fast instead, so they waste far less.
Draw your design, test it, then improve it: Is the dimming smooth? Is the rod warm? Does the lamp reach full brightness?
Try it
Predict, then check. (1) Mark the lead every 2 cm. Predict the brightness at each mark, then check by eye. (2) Replace the lead with a thicker one. (3) Use only two batteries instead of four. (4) Feel the rod after a minute: warm or cold? Why? Write one sentence for each.
Key formulas and definitions
- I = V ÷ (R_lamp + R_rod) (series circuit)
- R = ρ × L ÷ A (length, area)
- P = I² × R (power in the lamp)
- Longer rod, thinner rod: more R, less I, dimmer lamp
Worked examples
1. A 6 V battery, a 6 Ω lamp and a 6 Ω rod are in series. Find the current.
Total R = 6 + 6 = 12 Ω. I = 6 ÷ 12 = 0.5 A.
2. For the same circuit, find the power in the lamp.
P = I² × R = 0.5² × 6 = 1.5 W. With no rod it would be 1 A and 6 W, so the lamp is at 25%.
3. A 12 cm length of pencil lead has 4 Ω. What is the resistance of 30 cm of the same lead?
R is proportional to length. 30 ÷ 12 = 2.5, so R = 2.5 × 4 = 10 Ω.
4. A rod of 18 Ω is fully used with a 6 Ω lamp on 6 V. Find the lamp power.
I = 6 ÷ 24 = 0.25 A. P = 0.25² × 6 = 0.375 W. That is about 6% of full power.
5. A thick rod has half the resistance of a thin rod of the same length. A thin rod of 12 Ω is replaced by the thick one. What is the new R?
Doubling the area halves R: 12 ÷ 2 = 6 Ω.
6. In example 2, how much power does the whole circuit take, and how much heats the rod?
Total P = V × I = 6 × 0.5 = 3 W. Lamp 1.5 W, rod = 0.5² × 6 = 1.5 W as heat. Half is wasted as heat.
Common mistakes
- Joining the rod in parallel with the lamp. A dimmer must be in series so all the current passes through both.
- Thinking the lamp turns fully off at the end of the rod. The rod still has a limited resistance, so a small current flows.
- Using a very hard (H) pencil lead. It is too resistive and the lamp may never light well. Use a soft 2B or 4B lead.
- Forgetting that the rod wastes energy as heat. A series dimmer does not save electricity in proportion to the dimming.