How to use Magnetic Field Sensor?

I can't figure out how to use the Magnetic field sensor, I may be completely dumb but take a look at this:

import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorManager;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;

Context context;
SensorManager manager;
Sensor sensor;
float X1;
float Y1;
float Z1;

void setup() {
  fullScreen();

  context = getActivity();
  manager = (SensorManager)context.getSystemService(Context.SENSOR_SERVICE);
  sensor = manager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

 // manager.registerListener(listener, sensor, SensorManager.SENSOR_DELAY_GAME);
}
public void onSensorChanged(SensorEvent event)
     {
       X1 = event.values[0];
       Y1 = event.values[1];
       Z1 = event.values[2];
     }

void draw() {


  text(str(X1) + " " + str(Y1) + " " + str(Z1) , width/2, height/2);
}

All I get is 0.0, 0.0, 0.0 (And my phone does have the sensor) Does anyone know the proper way of doing this?