If electricity can make magnetism, can magnetism make electricity? Faraday kept a note to try. For years nothing worked. A magnet sitting beside a coil made no current at all.
On 29 August 1831 Faraday wound two separate coils of insulated wire on an iron ring about 6 inches across. One coil went to a battery, the other to a galvanometer, a needle that swings when current flows. When he connected the battery, the needle kicked, then went back to zero. When he disconnected it, the needle kicked the other way. A steady current did nothing. Only a change made a current. That autumn he also pushed a bar magnet into a coil and saw the needle swing, and then turned a copper disc between the poles of a strong magnet and drew a steady current from it, an early generator.
Key idea
Flux, $\Phi$, measures how much magnetic field passes through a loop: field strength times area, $\Phi = BA$, in webers. When a magnet slides into a coil, the flux through it rises; when it slides out, the flux falls, and the voltage flips sign. Shake a magnet back and forth and you make alternating current, the kind power stations make with huge spinning magnets and coils.
Why it works
Let's think it through
Worked example: Faster and more turns
- Twice as fast: twice the rate of change, so twice the voltage.
- Twice the turns: twice again.
Answer: $0.3 \times 2 \times 2 = 1.2$ V.
Worked example: Enough for an LED?
- $1.8 \text{ V} = 1800$ mV.
- $1800 \div 2 = 900$ turns.
Answer: At least about 900 turns, with a strong magnet and a fast shake.
Common mistake
Build it: a shake generator and a compass meter
- Thin magnet wire, about 30 gauge, 50 to 100 meters (a spool is about \$8 to \$12). More turns are better.
- A clear plastic tube or an empty pill bottle that a stack of magnets slides through easily, about 1.5 to 2 cm wide inside
- 3 to 5 neodymium disc magnets that fit the tube (about \$5 to \$8)
- Two caps or tape to close the tube ends
- For the meter: a small compass and 1 meter of any insulated wire; or an inexpensive multimeter set to its lowest DC or AC voltage range
- Optional challenge: two red LEDs
- Sandpaper and tape
- Put the magnets inside the tube and close both ends so they cannot fall out.
- Wind the magnet wire around the middle of the tube, all in one direction, in a band about 2 to 3 cm wide. Aim for at least 500 turns. Count in hundreds, or have a helper count. Tape the coil.
- Sand the two wire ends until the copper is bright.
- Make a compass meter: wind 10 to 20 turns of insulated wire around the compass, so the coil runs north and south over the needle. Set it on the table at least an arm's length from the magnets, so they don't pull the needle themselves.
- Connect the two coils together. Shake the tube hard, or slide the magnets in with one quick push. Watch the needle.
- Challenge: connect two LEDs side by side across the coil, facing opposite ways (long leg of one to short leg of the other). Shake hard in a dim room.
What to look for
- The compass needle kicks one way when the magnets rush toward the coil's center, and the other way as they leave.
- When the magnets stop, the needle goes back to rest. No change, no current.
- With enough turns and a hard shake, the LEDs flicker, taking turns: proof that the current alternates.
Troubleshooting
- Needle doesn't move? Check that the enamel is sanded off and the connections are tight. Move the compass farther from the magnets.
- Very small kick? Shake faster, add turns, or use more magnets.
- No LED glow? Most homemade coils need many hundreds of turns. A multimeter shows how close you are.
Iterate like Faraday
- Measure how the kick changes with speed: drop the magnets through the coil from different heights and compare.
- Count turns: make a second coil with half as many and compare the kick.
- Faraday kept a careful notebook and numbered every paragraph. Number yours: setup, change, result, for each try.
- Strong (neodymium) magnets snap together hard and can pinch skin. Slide them apart instead of pulling. Keep them, and any small batteries, away from small children: swallowed magnets and button or coin batteries can cause serious injury.
- Keep the magnets well away from the compass you are using as a meter, or it will point at them instead.
- A grown-up does the cutting and stripping of wire, and any work with a craft knife.
- This project uses no battery: the coil makes all the electricity. Keep it that way.