12.2.4 Hertz's Discovery of Radio Waves

• Heinrich Hertz discovered the existence of radio waves
  • He made a short air gap between wires and put a large potential difference across this gap, so high voltage sparks bridged the gap
  • These sparks generated radio waves, so this was a radio wave transmitter
• Hertz detected these radio waves with two pieces of equipment:
  • A circular wire with a small break in the circuit produced sparks across the break when held near the source of radio waves
  • A concave metal sheet with two parallel metal rods at the centre which had oscillating potential difference induced across them by the radio waves' alternating electric field

Equipment used by Hertz to detect radio waves emitted from a transmitter

The high voltage spark gap transmitted radio waves and the detector received these. The detector shown here is the two parallel metal rods surrounded by a concave metal sheet, but Hertz also used an incomplete wire ring which formed sparks across the gap when detecting radio waves. 

• Hertz showed the waves could be reflected:
  • He placed a metal screen behind the source and measured a stronger signal with the detector
  • This showed some radio waves were reflected off the screen and back towards the detector
• Hertz showed the waves were able to penetrate insulators:
  • When an insulator was placed between the transmitter and detector, there was no difference in the signal detected
• Hertz showed the waves were polarised:
  • When the detector was rotated 90° perpendicular to the path of the radio waves, sparks stopped being produced in the detector
  • This showed the electric fields of the radio waves were only oscillating in a single plane
  • The cause for this was that electrons were only being accelerated in one direction so the radio waves were all polarised in the same plane
• Perhaps most crucially, Hertz measured the speed of the radio waves:
  • He reflected radio waves from the transmitter off of a flat metal sheet
  • This produced a standing wave
  • When passing a detector across the region containing the standing wave, a large signal was detected at antinodes, and no signal was detected at nodes
  • This allowed Hertz to find the wavelength and he knew the frequency by using the properties of the transmitter circuit
  • He used the wave equation (v = fλ) to determine the speed of the waves
  • This speed was very close to the value calculated by Maxwell, showing that radio waves are electromagnetic waves

Standing wave to determine speed of radio waves

Here, the second type of detector is shown - an incomplete ring of wire which forms sparks. Hertz passed this along the standing wave and located antinodes using the strongest sparks. The distance between antinodes represents half the wavelength of the radio wave.