When we think of forces, we usually imagine a push or a pull. Such a push or pull is assumed to be in the direction of the line connecting the pusher and the object pushed. Not so with the Lorentz force. A magnetic field acting on a moving charge pulls in a direction perpendicular to the velocity of the particle and perpendicular to the magnetic field. If the particle moves parallel to the field, there is no force. If the particle comes to rest, there is no force. Finally, if the particle were to lose its charge, there would be no force. This chapter requires finding a magnetic field that can focus charged particles.