June
2006, Issue 191
Earth
Field Magnetometer
Cypress
PSoC High Integration Challenge 2004 Winner
MAGNETISM
101
A
brief overview of magnetic terms may be helpful before
diving any deeper. The magnetic units I’ll refer to
in this section belong to the centimeter-gram-second
(CGS) measurement system.
A
current flowing through a coil of a wire creates a magnetic
field at its center. This has a field strength (H) measured
in oersteds (Oe):
where
I is the current in amps, N is the number of turns,
and L is the coil length in centimeters.
Magnetism
inside material is measured by its flux density (B)
measured in gauss. Ferrous materials tend to pull in
more than their share of the nearby field. The flux
lines are concentrated or increased within the material,
forming an hourglass shape. The amount of concentration
is given by the relative permeability (µR). Thus, B
= µR × µ0 ×
H, where the permeability of free space µ0 = 1.
A
ferrous material has a limit on how much magnetic flux
it can accept. The material is saturated above that
limit. Increasing H will no longer cause B to increase.
The saturation characteristic is important enough that
datasheets for inductor core materials always include
a plot of B versus H, which is known as a hysteresis
loop.
Nonferrous
materials don’t saturate and have a relative permeability
(µR) of 1, meaning they won’t affect the field. Therefore,
B = H, or a 1-Oe field produces 1 G. This can cause
some confusion, because the field strength of permanent
magnets (including the Earth) is given in gauss rather
than oersteds. However, it isn’t an issue here because
they’re equivalent in air.
The
SI unit of flux density is a tesla (T), where 1 T =
10,000 G. It’s customary to use the nanotesla (nT),
also known as a gamma, when talking about magnetic storms
because the unit size is more convenient.
