Electromagnetic radiation travels (propagates) by means of oscillating electromagnetic fields that pass through the air and the vacuum of space equally well, and does not require a medium of transport (such as the aether). When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. The word 'radio' is used to describe this phenomenon, and television, radio, radar, and cell phone transmissions are all classed as radio frequency emissions.