Hertz Proves Radio Waves Exist (1887): Maxwell's Theory Confirmed

In 1887 the German physicist Heinrich Hertz, working at the Technische Hochschule in Karlsruhe, generated and detected electromagnetic waves in his laboratory for the first time. Over the following months he showed that these invisible waves travel at a finite speed and can be reflected, refracted and polarised like light. His experiments confirmed James Clerk Maxwell’s theory of electromagnetism and laid the foundations for radio, television and wireless communication.

Background: Maxwell’s prediction

In the 1860s the Scottish physicist James Clerk Maxwell combined the laws of electricity and magnetism into a single set of equations (see Maxwell’s electromagnetic theory). They predicted that changing electric and magnetic fields could travel through space as waves at the speed of light, and that light itself was such a wave.

Maxwell’s theory was elegant but unproven, and it was more widely accepted in Britain than on the Continent, where rival theories of electrical action at a distance were popular. In 1879 the Berlin Academy of Sciences, encouraged by Hermann von Helmholtz, offered a prize for experimental evidence on the question. Helmholtz suggested the problem to his gifted student Heinrich Hertz, who at first judged it too difficult.

Heinrich Hertz

Heinrich Rudolf Hertz was born in Hamburg on 22 February 1857. He studied in Munich and Berlin, where he earned his doctorate under Helmholtz in 1880. After posts in Berlin and Kiel, he became professor of physics at the Technische Hochschule in Karlsruhe in 1885. There he had a well-equipped laboratory and the freedom to return to the problem Helmholtz had set him.

The experiments

In autumn 1886 Hertz noticed that a discharge from an induction coil through one coil of wire could produce sparks in a nearby, unconnected coil. He built on this observation. His transmitter, or oscillator, was a pair of straight wires ending in metal spheres or plates, separated by a small spark gap. When a high voltage from an induction coil jumped across the gap, the current surged back and forth extremely rapidly, many millions of times a second.

His detector was simply a loop of wire, also with a tiny gap. When the waves from the oscillator reached the loop, they induced a current that caused faint sparks to jump across its gap. Hertz often had to observe them in a darkened room, sometimes with a magnifying lens.

In 1887 he published his paper “On very rapid electric oscillations”, describing how he could produce and detect these effects across a room. He went on to set up standing waves by reflecting them off a zinc sheet on a wall, which allowed him to measure their wavelength. Combined with the calculated frequency of his oscillator, this gave a speed of the same order as the speed of light.

In 1888 Hertz used large curved metal mirrors to focus the waves into beams. He showed they could be reflected, refracted through a large prism of pitch (a tar-like material) and polarised with a grid of parallel wires, behaving in every way like light but with much longer wavelengths. His results appeared in a series of papers in 1887-1889 and were collected in a book, translated into English as Electric Waves in 1893.

The photoelectric effect

During the 1887 experiments Hertz noticed that the sparks in his receiver jumped more easily when ultraviolet light fell on the gap. He reported the effect but did not explain it. This was an early observation of the photoelectric effect, later explained by Albert Einstein in 1905 in terms of particles of light.

From physics to radio

Hertz died in Bonn on 1 January 1894, aged only 36. He saw his work as a contribution to pure physics, and an often-repeated story, based on later recollections, says he saw no practical use for his waves.

Others quickly saw the possibilities. In 1894 Oliver Lodge in Britain demonstrated the reception of “Hertzian waves” with an improved detector, and Augusto Righi in Italy and Jagadish Chandra Bose in India pushed experiments to shorter wavelengths. The young Italian Guglielmo Marconi, who had studied Hertz’s work, began building wireless telegraphy equipment in 1894-1895 and in 1901 claimed the first transatlantic radio signal.

The unit of frequency, one cycle per second, was named the hertz in his honour, and it became part of the International System of Units in 1960.

Timeline

• 1857, 22 February: Heinrich Hertz is born in Hamburg.
• 1879: The Berlin Academy offers a prize for evidence on electromagnetic theory.
• 1885: Hertz becomes professor of physics at Karlsruhe.
• 1886, autumn: Hertz observes sparks induced between separate circuits.
• 1887: Hertz publishes “On very rapid electric oscillations” and observes the photoelectric effect.
• 1888: He shows the waves travel at a finite speed and can be reflected, refracted and polarised.
• 1889: Hertz moves to the University of Bonn.
• 1894, 1 January: Hertz dies in Bonn.
• 1901, 12 December: Marconi reports receiving a radio signal across the Atlantic.

Why it matters

Hertz turned Maxwell’s equations from theory into experimental fact, proving that light is one member of a much wider family of electromagnetic waves. His discovery underlies radio, radar, television, mobile phones and Wi-Fi, all of which use the waves he first detected in a Karlsruhe laboratory.