Variable Inductor

PARTS AND MATERIALS

• Paper tube, from a toilet-paper roll
• Bar of iron or steel, large enough to almost fill diameter of paper tube
• 28 gauge “magnet” wire
• Low-voltage AC power supply
• Incandescent lamp, rated for power supply voltage

CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 14: “Magnetism and Electromagnetism”
Lessons In Electric Circuits, Volume 1, chapter 15: “Inductors”
Lessons In Electric Circuits, Volume 2, chapter 3: “Reactance and Impedance—Inductive”

LEARNING OBJECTIVES

• Effects of magnetic permeability on inductance.
• How inductive reactance can control current in an AC circuit.

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

Wrap hundreds of turns of magnet wire around the paper tube. Connect this home-made inductor in series with an AC power supply and lamp to form a circuit. When the tube is empty, the lamp should glow brightly. When the steel bar is inserted in the tube, the lamp dims from increased inductance (L) and consequently increased inductive reactance (XL).

Try using bars of different materials, such as copper and stainless steel, if available. Not all metals have the same effect, due to differences in magnetic permeability.