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Illustration of the apparatus used by Heinrich Rudolf Hertz (1857–1894) to discover radio.
Illustration of the apparatus used by Heinrich Rudolf Hertz (1857–1894) to discover radio.
Electromagnetism, Maxwell’s equations, and light were all cornerstones of scientific experiment and research in the late nineteenth century. At the same time that Michelson and Morley were carrying out their experiments in what proved a futile attempt to find evidence for the ether, the German physicist Heinrich Hertz was making a breakthrough which not only provided further proof of the accuracy of Maxwell’s theory, but opened the way to modern communications.
When Maxwell found that the equations that describe electromagnetic waves determined that these waves must travel at the speed of light, he not only concluded that light must be a form of electromagnetic wave, but predicted that ‘other radiations’ could be ‘propagated through the electromagnetic field according to electromagnetic laws’ (see here). It was these other radiations, now called radio waves, that Hertz discovered in the mid 1880s.
Following in the footsteps of Michael Faraday (see here), Hertz had been carrying out experiments involving induction using a pair of Reiss spirals – coils of wire wound like a solenoid with their ends connected to metal balls separated by a small gap. He noticed that when electricity from a storage device known as a Leyden jar was discharged into one of these coils (via the metal balls), a spark jumped across the gap in the other coil, even though the two spirals were not physically connected and were not – as in Faraday’s coils – wrapped around a shared iron rod. Some influence had travelled through the space between the spirals to make the second one spark in response to the activity of the first one.
In order to investigate the phenomenon, Hertz scaled up his experiment. He used an electric generator and an induction coil to make sparks not in a Reiss spiral but across a gap in the middle of a piece of wire two metres long, ending in spheres 4 millimetres in diameter. The long wire acted as a radiator for electromagnetic energy (in modern language, the antenna of a transmitter). The electric current associated with the sparks bounced to and fro along the wires. Various pieces of equipment were used to detect the waves being radiated by the antenna, the simplest being a piece of copper wire, 1 millimetre thick, bent into an almost complete circle 7.5 centimetres across. There was a small brass sphere on one end of the wire; the other end was pointed, nearly touching the sphere. The adjustable gap between the point and the sphere was typically a few hundredths of a millimetre. Sparks leaping across the gap in the transmitter produced corresponding sparks in the receiver, and by moving the receiver to different distances from the transmitter, over a range of a few metres, and measuring the strength of the response, Hertz could measure properties of the waves being radiated. He also carried out experiments in which a zinc plate was used as a reflector to generate standing waves for study.
In a series of experiments carried out between 1886 and 1889, Hertz found that the waves were four metres long, twice the length of the transmitting antenna, and travelled at the speed of light. He showed that these waves they could be reflected and refracted like light, and focused by concave reflectors. This was a dramatic confirmation of Maxwell’s theory of electromagnetism, since visible light has wavelengths in the range 400 nanometres to 700 nanometres, where a nanometre is a billionth of a metre. The Hertzian waves were roughly ten million times bigger than light waves, but obeyed the same rules.
All of the discoveries were published in a series of papers in the journal Annalen der Physik, before being in presented Hertz’s book, Untersuchungen Ueber Die Ausbreitung Der Elektrischen Kraft (Investigations on the Propagation of Electrical Energy), in 1892.
The book is now considered a classic, but Hertz had no idea of the practical implications of his discovery. He told a student that:
‘It’s of no use whatsoever; this is just an experiment that proves Maestro Maxwell was right – we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.’
‘What next?’ asked the student.
Hertz replied,
‘Nothing, I guess.’25
He was wrong. The waves he discovered – now called radio waves – revolutionized communications in the twentieth century, laying the foundations of radio, radar, TV and the wireless society on which we now depend. Appropriately, the modern unit of frequency (cycles per second) is named the Hertz (Hz) in his honour. In those units, the waves investigated by Hertz had a frequency of a few hundred Megahertz (MHz), generated by a half-wave dipole antenna.