Animation of a half-wave dipole antenna transmitting radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods, with an alternating current applied at its center from a radio transmitter (not shown). The voltage charges the two sides of the antenna alternately positive (+) and negative (−). Loops of electric field (black lines) leave the antenna and travel away at the speed of light; these are the radio waves.
Michigan Technological University Ion Space Propulsion Laboratory. Upper Left: Xenon Hall Thruster, (The Dawn spacecraft has these) Upper Right: Bismuth Hall Thruster, Lower Left: Magnesium Hall Thruster, Lower Right: Zinc Hall Thruster. More: http://www.me.mtu.edu/researchAreas/isp/