The magnetic sensor in the Mote is the Honeywell
HMC1002 magnetometer, which is a magnetoresistive
sensor. The anisotropic magnetoresistive (AMR) sensor has a wide range for sensing the Earth’s magnetic field and
provides both the strength and direction of the Earth field
[6]. Figure 4 shows the magnetoresistive effect of
Permalloy. The resistance of Permalloy is a function of the
angle (θ) between the bias current and the magnetization
vector (M ). The applied magnetic field changes the
direction of the magnetization vector and thus changes the
resistance, which is used for magnetic field sensing. The
AMR sensor is made of a nickel-iron (Permalloy) thin film
deposited on a silicon wafer and patterned as resistive
strips. Typically, four of these resistive strips are connected
in a Wheatstone bridge configuration so that both
magnitude and direction of a field along a single axis can be
measured. The key benefit of AMR sensors is that they can
be bulk manufactured on silicon wafers and mounted into
commercial integrated circuit packages