By johnmcgee
Updated Mar 24, 2022
Electromagnets harness the magnetic field produced when electrons flow through a coil of wire. By winding the wire tightly and adding a magnetic core, the field’s strength can be amplified. While a constant DC supply creates a steady pull, many applications—such as loudspeakers—require a field that changes with the signal. The simplest way to make a variable‑strength electromagnet is to control the current with a rheostat (variable resistor).
Use an old dial‑type dimmer or a dedicated rheostat. Confirm that it has only two terminals; a potentiometer will have three and is unnecessary for this circuit. A two‑terminal resistor keeps the wiring simple.
Test a piece of metal such as a screwdriver or a large bolt by holding a kitchen magnet close. If it attracts, the metal is ferromagnetic and suitable for the core.
Wrap insulated copper wire around the core for several hundred turns. Overlap the turns if needed. Leave about 12 cm (half a foot) of wire free at each end.
Strip the insulation from both ends of the wire. Tape one end to a terminal of a 9‑V battery. Secure the other end to one terminal of the rheostat. Repeat with the second wire, connecting the other end to the remaining rheostat terminal and the battery’s opposite terminal.
Use the electromagnet to pick up paper clips or other small ferrous objects. Turn the rheostat dial to vary the current; observe how the magnetic field strengthens or weakens as you adjust.
An AC electromagnet is possible, but a DC electromagnet—like the one described—offers stronger, more controllable magnetic fields for the same voltage.
