CathMag is an educational physics application for iOS devices. It takes advantage of the built-in magnetometers on many of the devices. The screen simulates a cathode ray tube that can emit a variety of particles traveling at different speeds. The behavior of these particles is determined by the magnetic field that surrounds the iOS device. That magnetic field can be altered with a strong magnet. When the user moves the magnet closer or further away from the iOS device, the charged beam will bend more or less to the left or right (depending on the selected particle charge and velocity.