In 1861, the Scottish physicist, James Clerk Maxwell, summarised everything that was known at the time about electricity and magnetism into four equations.
Using the four equations, Maxwell predicted the existence of electromagnetic waves with varying wavelengths and frequencies but with the same speed as visible light which was known to be about 300 000 km/s.
To produce a radio wave, Hertz used a high voltage induction coil that produced an AC voltage signal with a frequency of about 300 million hertz. The high voltage made it possible charges to cross a narrow gap between between two brass spheres at the ends of a semi-circular conducting loop, creating sparks.
The equipment used by Hertz to generate and detect a radio wave.
To detect the electromagnetic waves some distance away, he used another semi-circular conducting loop with brass spheres as a receiver. As sparks jumped across the gap in the source circuit, they were also observed jumping across the gap in the receiver loop.
Hertz reasoned that the spark discharge from the gap in the source circuit produced an electromagnetic wave. When these waves arrived at the receiver, the varying electric field in the wave caused the charges in the receiver loop to oscillate as well, producing the same sparking across the gap.
Hertz also wanted to show that these waves had the same speed as visible light. He measured their speed by setting up a standing wave pattern. The waves from the source reflected off a zinc plate and superimposed to produce a standing wave.
Measuring the wavelength of radio waves to calculate their speed.
Hertz moved the receiver coil along the length of the standing wave. The spark across the gap was most intense at the anti-nodes and did not exist at the nodes. The distance between two adjacent anti-nodes was half a wavelength so the wavelength could be determined. The frequency was known by Hertz so the speed could be easily calculated using the wave equation. It turned out to be very close to 300 000 km/s, as predicted by Maxwell.
Hertz' experiments confirmed Maxwell's predictions about electromagnetic waves and provided some good experimental evidence for the idea that visible light was a form of transverse electromagnetic wave. The waves produced by Hertz eventually became known as radio waves and his research led to the development of radio communications.
While doing his initial experiments, Hertz noticed that the sparks jumping across the gap in the receiver loop were more intense when it was exposed to bright light. Although this was an incredible discovery, Hertz didn’t think much of it and confined his research to the production and study of electromagnetic waves. What Hertz had discovered, but chose not to investigate, was the photoelectric effect. This phenomenon would one day make solar energy an important source of electrical power for society.