In its frequent incarnation as a physics demonstration, a Gauss gun usually consists of series of ferromagnetic balls on a nonmagnetic track. On the track is a permanent magnet with a ball, the projectile, stuck to the front of it. Between the projectile and the magnet is a spacer, usually consisting of one or more additional balls. Yet another ball, the trigger ball, is released from behind the magnet. It is attracted to and accelerates toward the magnet. When it strikes the back of the magnet, it transfers its momentum to the projectile ball, which is knocked off the front of the stack, as in a Newton's cradle. Because the spacer kept it far away from the magnet, the projectile loses less energy escaping from the magnet's influence than the trigger ball gave it, so it leaves the stack with a higher velocity than the trigger ball entered with.