Hand Cranked AC Generator

Observe the induced current in a gimbaled coil as it rotates in Earth's magnetic field.

What it Shows

A changing magnetic flux through a circular coil of wire induces a current in the wire. By spinning a circular coil of wire at constant frequency and measuring the induced voltage across its ends we can find the local direction and magnitude of the Earth's magnetic field as it passes through the coil. The commutators of the coil are configured to produce an alternating current.

Hand Cranked AC Generator

How it Works

The coil is mounted inside a wooden gimbal that can be spun with a handle at a more or less constant frequency. The coil is free to orient in any direction, and so the amplitude of the AC signal can be interpreted in terms of the local direction of the Earth's magnetic field. The ends of the coil's wire are merged into a single coaxial BNC cable that allows the singal to be passed via a low-pass amplifier to an oscilloscope.

Hand Cranked AC Generator Hand Cranked AC Generator


  • The Earth's magnetic field in the Boston area points into the ground at an angle of 68 degrees below a line pointing to magnetic north (http://www.ngdc.noaa.gov/geomag/icons/wmm2000i.gif) with a magnitude of about 5.4x10-5 Tesla (http://www.ngdc.noaa.gov/geomag/icons/wmm2000f.gif). Compass needles can be provided to show the direction of magnetic north.
  • The coil has a radius of 15 cm (area = 0.07 m2) and is wound with 60 turns of wire.
  • The maximum induced peak-to-peak voltage at 2 Hz is roughly 4.5 mV. The coil's resistance is about 1 Ohm.
  • The EG&G Model 189 Selective Amplifier filters signals above 10 Hz (settings: Q=1; Freq=10.00Hz; input coupling=DC; preamp gain=x10).