When measuring the magnetic field using a sensor in a smartphone, one can choose from several applications1–3 with useful graphical interfaces. For my first endeavor, I tried to locate the magnetic field sensor (MFS) in my smartphone SC-04D. I was able to find plenty of pictures on the internet of my phone dismantled, but none of them helped me locate the MFS. I did not want to risk dismantling my own phone, so I used a piece of weakly magnetized iron and found a point where the magnetic field reading was the highest (188 µT) with the application Sensor Box for Android.1 I then assumed the sensor was just below the surface of that point (Fig. 1). In an earlier paper,4 Silva introduced readers to the app 3D Compass and Magnetometer2 and “Relative Mode,” which can be used to “zero out” background noise such as...

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