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History

Hertz, Radio Waves, and the Photoelectric Effect

A spark transmitter proves electromagnetic waves exist — and light unexpectedly changes another spark

James Clerk Maxwell's electromagnetic theory predicted that changing electric and magnetic fields could travel through space as waves. In the late 1880s, Heinrich Hertz built apparatus that could generate and detect those waves in the laboratory.

For the first time, an electrical signal did not need a wire between transmitter and receiver.

The spark-gap transmitter

Hertz used a rapidly changing high-voltage discharge to excite oscillating electrical currents. A spark jumping a gap produced a burst of high-frequency electromagnetic energy.

His receiving apparatus could be remarkably simple: a loop of conductor with a very small gap. When the transmitted electromagnetic wave reached the loop, a tiny spark could appear across the receiver gap.

high-voltage spark → rapidly changing current → electromagnetic wave → receiving loop → tiny spark

Radio waves become a laboratory fact

Hertz showed that the waves could reflect, interfere, and behave in ways expected from Maxwell's theory. These experiments did not yet make a practical radio communication system, but they established the physical basis for one.

Then light changed the spark — 1887

While working with spark gaps in 1887, Hertz noticed that ultraviolet light falling on an electrode made a spark occur more readily. The observation was one of the first experimental sightings of what would become known as the photoelectric effect.

In 1888, Wilhelm Hallwachs studied the effect further and showed that ultraviolet light could cause a negatively charged metal surface to lose charge.

The surprising idea:

Light is not only something we see. Under the right conditions, light can cause electrical charge to leave a material.

The full explanation would take much longer. In 1905, Albert Einstein would explain the photoelectric effect using quantized packets of light energy. For our historical device trail, however, the key point is that by 1887 experimenters had already found a direct connection between light and released electric charge.

This will eventually create light-sensitive components

Photoelectric cells, phototubes, photodiodes, camera sensors, optical encoders, and solar cells all belong to later branches of the same broad idea: light can alter electrical behavior or create electrical carriers.

Measurement trail 18 — frequency, wavelength, and the hertz

A repeating electromagnetic wave can be described by its frequency, measured today in hertz (Hz), and its wavelength.

c = fλ

where c is the speed of light, f is frequency, and λ is wavelength.

A 100 MHz radio wave

f = 100 MHz = 100,000,000 Hz

λ ≈ 300,000,000 m/s / 100,000,000 Hz = 3 m

So a 100 MHz electromagnetic wave has a wavelength of roughly 3 meters in free space.

A receiver still needs a better detector

Hertz's tiny spark was enough to prove the waves existed, but it was not an especially sensitive or convenient receiver. Practical wireless communication would need a device that could respond reliably to much weaker signals.

The coherer will soon provide one early answer. At nearly the same time, AC engineers are solving a different problem with another elegant electromagnetic device: the induction motor.

< Alternating Current Becomes Useful | History Index | The Induction Motor >