The Vacuum Tube

 It is a glass tube surrounding a vacuum. It is interesting because when electrical contacts are put on the ends, you can get a current to flow through that vacuum. Thomas Edison first saw this in 1883 and he discovered that electrical current doesn't need a wire to function.

Lee De Forest then created the "audio" which made it so the currents would move in a single direction and this would cause currents to be much stronger and they would come out the main electrodes of the tube.