Magnetic

I would like to the Magnetic Sensor, but no changed the sensor at NexusOne.
If I run anywhere, the sensor change ?

I tried source code bellow:

Get SensorManeger:

mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);

Get Sensor:

mMagnetic = mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);

Implement SensorEventListener:

public class MagneticActivity extends Activity   implements SensorEventListener{

@Override
public void onSensorChanged(SensorEvent event) {
 Log.d("test", "onSensorChanged:" + event.timestamp + ":" + "x=" + event.values[0] + "y=" + event.values[1] + "z=" + event.values[2]);
}
  

source code here