June 2006, Issue 191

Earth Field Magnetometer
Cypress PSoC High Integration Challenge 2004 Winner

HELMHOLTZ COIL

The coil assembly is shown on the right-hand side of Photo 1. The coil consists of a pair of windings in series that’s axially separated by a distance equal to the windings’ radii. All three derivatives of the magnetic field strength go to zero at its center. The area of uniform field is approximately half its radius in size. The flux density is:

where B is in nanoteslas. N is the number of turns in each winding. I is the current in amps. R is the radius in meters. I chose a 1.875² radius for the coils and used 217 turns, which produced a flux density of 4,096 nT per milliamp. This requires a current of about 7 mA to cancel out the Earth’s 30,000-nT field.