Edwin H. Hall, an American physicist in the 19th century, discovered the Hall effect which allows for the measurement of magnetic fields. The Hall effect occurs when a conductor carrying an electric current is placed in a magnetic field perpendicular to the current. This interaction produces a potential difference on the sides of the conductor. Applications of the Hall effect include sensors used in doors, cars, computers, and anemometers. The document then discusses how Hall sensors can detect the polarity and strength of magnetic fields and how they are used to build circuits to detect magnet polarity. It also describes how mobile phone apps use Hall sensors to measure the Earth's magnetic field.