Appleton experimented transmitting to a receiver 70 miles away, varying wavelengths of the signal to measure reflections from the atmosphere. He found that the Kennelly-Heaviside layer was 50 miles high and most active at night. He also discovered a higher charged layer called the Appleton layers which were 150 miles high.
A visual chronology of Asimov's ~1500 scientific inventions and discoveries.
