A beam of electrons moves in a path, bent into a circle by a perpendicularly applied magnetic field.
The file was copied from http://commons.wikimedia.org/wiki/File:Cyclotron_motion_wider_view.jpg and modified by S. Zurek.
cyclotron_motion_wider_view.jpg
You can use this image freely, but you must give the following credits:
by Marcin Białek, CC-BY-SA-3.0