Hertz and the Detection of Radio Waves
Maxwell's theory was a beautiful piece of mathematics, but mathematics alone doesn't confirm physics. It took Heinrich Hertz, crackling a spark across a lecture theatre in 1887, to prove electromagnetic waves were real — and Hippolyte Fizeau, decades earlier, spinning a toothed wheel on a hillside, to pin down the speed they had to match.
Why Maxwell's theory still needed direct proof
Maxwell's electromagnetic theory made a remarkable prediction — that oscillating electric and magnetic fields could propagate through space as self-sustaining waves, and that light itself must be one example of this phenomenon. But the theory as it stood in the 1870s was purely mathematical. Nobody had ever deliberately generated an electromagnetic wave that wasn't visible light, or shown directly that such a wave carried energy across empty space, reflected, refracted and polarised the way light does. Until that direct demonstration existed, Maxwell's theory — however elegant — remained an unconfirmed prediction rather than an established fact.
Hertz's spark-gap experiment
Heinrich Hertz set out, in 1887, to generate and detect an electromagnetic wave at a distance using purely electrical apparatus, with no optical equipment involved at all.
Purpose: to test Maxwell's prediction directly, by generating and detecting an electromagnetic wave that wasn't visible light, and comparing its properties and speed against those of light. Method: Hertz built a transmitter consisting of two metal rods with a small spark gap between them, connected to a source of rapidly oscillating high voltage; each time the voltage was high enough, a spark jumped across the gap, and the rapid oscillation of charge this produced radiated an electromagnetic wave outward. A short distance away, Hertz set up a separate receiving loop of wire with its own small spark gap and no electrical connection to the transmitter at all; if an electromagnetic wave reached the loop, it should induce an oscillating current in it, large enough to produce a small spark visibly jumping across the receiver's own gap. By moving the receiver to different positions in the room and noting where the induced sparks were strongest and weakest, Hertz could also map out the wave's pattern and measure its wavelength, and hence (knowing the oscillation frequency of the transmitter) calculate its speed. Result: sparks were observed jumping across the receiver's gap in step with the transmitter, showing a wave had genuinely been transmitted and detected with no physical connection between the two circuits; the waves were also shown to reflect off metal sheets, refract through prism-like insulating blocks, and be blocked or transmitted depending on the orientation of a polarising grid, exactly as light does; and the measured speed came out matching $c$, the speed of light. Conclusion: these waves (now called radio waves) are a real, physically detectable phenomenon, sharing every characteristic property of light except frequency and wavelength — direct experimental confirmation that light and radio waves are both electromagnetic waves, exactly as Maxwell's theory predicted.
Hertz's result did two things at once: it proved electromagnetic waves genuinely exist and can be generated and detected under controlled laboratory conditions, and it showed that these waves behave identically to light in every measurable respect except frequency. Together, that turned Maxwell's mathematical prediction into an experimentally confirmed physical fact — and, as an incidental by-product, it also marked the birth of radio technology, though Hertz himself was chiefly interested in confirming the theory rather than in any practical application.
Fizeau's terrestrial measurement of the speed of light
Testing Hertz's measured wave speed against the accepted value of $c$, and testing Maxwell's calculated value of $c = 1/\sqrt{\mu_0\varepsilon_0}$ from the previous lesson, both required an accurate, independently measured value for the speed of light to compare against. Long before either Maxwell's or Hertz's work, in 1849, the French physicist Hippolyte Fizeau provided exactly that, using an ingenious purely mechanical method carried out entirely on the ground.
Until Fizeau's experiment, every measurement of the speed of light had relied on astronomical observations — timing effects involving the motion of Jupiter's moons or the apparent shift in starlight caused by Earth's own motion — which depended on assumptions about the solar system's geometry and scale. Fizeau's method needed none of that.
Purpose: to measure the speed of light directly, using apparatus entirely on Earth, independent of any astronomical observation or assumption. Method: Fizeau shone a narrow beam of light through a gap between the teeth of a rapidly rotating toothed wheel; the beam then travelled several kilometres across open ground to a distant mirror, reflected straight back, and returned through the same region of the spinning wheel to an observer. At low rotation speeds, the wheel had barely moved in the time the light took to make the round trip, so the returning light passed back out through the same gap it had left through, and could be seen clearly. As the rotation speed was steadily increased, the wheel moved further in that same round-trip time; at one particular speed, the wheel had rotated by exactly half a tooth-gap in the time light took to go out and come back, so a solid tooth (rather than a gap) now blocked the returning beam completely, and the light vanished from view. Result: from the known distance to the mirror, the known number of teeth on the wheel, and the precise rotation speed at which the light first disappeared, Fizeau calculated a value for the speed of light within a few percent of the modern accepted value — the first non-astronomical measurement of $c$ in history. Conclusion: light travels at a very large but finite, precisely measurable speed, and that speed can be pinned down using entirely terrestrial, mechanical apparatus, with no assumptions about the scale of the solar system required at all.
Fizeau's result mattered well beyond its own era. It gave physicists a trusted, independently derived numerical value for $c$ that later theoretical predictions — Maxwell's $1/\sqrt{\mu_0\varepsilon_0}$, and later experimental ones — like Hertz's measured radio-wave speed — could both be tested against. Without an accurate, independent value for $c$ already established, neither comparison would have been nearly as convincing.
Key takeaways
- Maxwell's electromagnetic theory was purely mathematical until it received direct experimental confirmation.
- Hertz (1887) used a spark-gap transmitter and a separate, unconnected spark-gap receiver to generate and detect electromagnetic waves (radio waves) at a distance.
- Hertz showed these waves reflect, refract and polarise like light, and measured their speed as matching $c$ — direct experimental proof of Maxwell's theory.
- Fizeau (1849) measured the speed of light using a rotating toothed wheel: light passing out through a gap, travelling to a distant mirror and back, is blocked by a tooth at the correct rotation speed.
- Fizeau's method gave the first accurate, purely terrestrial value for $c$, independent of astronomical observation.
- This independent value for $c$ was essential for comparing against both Maxwell's theoretical prediction and Hertz's later experimental measurement.
- Together, Maxwell's theory, Fizeau's measurement, and Hertz's experiment completed the case that light is an electromagnetic wave.