Magnetometer magnetometer = Magnetometer.GetDefault();

          if (magnetometer != null)
          {
              magnetometer.ReadingChanged += MagnetometerReadingChanged;
          }

private async void MagnetometerReadingChanged(Magnetometer sender, MagnetometerReadingChangedEventArgs args)
        {
            await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
               {
                   MagnetometerReading mg = args.Reading;

                   xAxis.Text = mg.MagneticFieldX.ToString();
                   yAxis.Text = mg.MagneticFieldY.ToString();
                   zAxis.Text = mg.MagneticFieldZ.ToString();

                   strength.Text =  Math.Sqrt(mg.MagneticFieldX * mg.MagneticFieldX +
                       mg.MagneticFieldY * mg.MagneticFieldY +
                       mg.MagneticFieldZ * mg.MagneticFieldZ).ToString();
               });
        }