How Heinrich Hertz Discovered Radio Waves

How was the first radio wave discovered and why?  Well, it all had to do with a pessimistic young German scientist named Heinrich Hertz and a contest that he was too intimidated to try.  This is one of the most influential experiments of all time.  Ready?  Let’s go. 

In 1879, Germany’s most famous scientist, Hermann von Helmholtz, had a great idea for the yearly “Berlin Prize” at the Prussian Academy of Science.  He suggested that they offer a prize for experimentally proving the existence of electromagnetic waves that travel in space, or to experimentally prove “the theory of electrodynamics which was brought forth by Faraday and was mathematically executed by Mr. Maxwell.” Helmholtz had a 22-year-old graduate student that he thought was perfect for the job, Heinrich Hertz.  Hertz thought about it but decided the problem was too difficult for him.  However, it planted the germ of the idea in his mind and he spent many years tackling the ideas of electromagnetic rays.  His diary had entire days where the comments were simply lines like:  “Thought about electromagnetic rays”, “Nothing but electromagnets”, “Worked on electromagnets all day”, “Electromagnets, still without success”, and my personal favorite because I identify with it too well “[realized] that most of what I have found so far is already known.”  He ended 1885 with “Happy this year is over, and hoping that it will not be followed by another like it.” 

Heinrich Hertz
Heinrich Hertz

It turned out 1886 was a far superior year, for both personal and professional reasons.  1886 was the year Hertz met a woman named Elisabeth Doll and fell in love.  Also, in the spring of that year, Hertz was showing his equipment to his new fiancée and happened to see a spark from a coil that was a distance from a discharging Leyden jar.  Hertz was then inspired to attempt the experiment that had intimidated him seven years earlier.  Finally, in September of 1887, Hertz found a way experimentally validated that light is an electromagnetic wave.  How did he do that?  Well, Hertz made a wave with vibrating electronics and then proved that his new invisible wave would travel across a room.  He eventually proved that his new wave would reflect off of mirrors like light, bends through prisms like light, and even moves at the speed of light!  Whoa.  What was this new invisible light?  Well, by the 1920s these waves, which were originally called Hertzian waves, were christened radio waves!

Side comment: when most people think “Radio,” they think of music, which is why many people think that radio waves are really sound waves.  This is wrong.  Radio waves are invisible low-frequency light waves.  To explain this better, think about records and record players.  Records are disks made of vinyl with little bumps in them.  When a needle bounces on the bumps it can be used to vibrate a cone and make music.  The record, therefore, is not made of music, it is made of vinyl.  Radio waves are also not made of music, they are made of electromagnetic waves like light, but they can have “bumps” in the waves that can be used to create sound. 

Anyway, back to Hertz, how did he make radio waves?  Well, luckily Hertz had a device called a spark gap generator, which would transform the power from a DC battery into bursts of very high alternating voltage and a large spark.  I will describe how a spark gap works and how it was created in detail in the next video.  Anyway, all Hertz had to do to create his invisible electromagnetic wave is to add antennae.  So, that is what he did, he added two wires to the spark generator with metal spheres at the end.  Now he could create waves, but he had to detect them as well.  For that, he made a circular receiver with a tiny little gap in them with the hope that if they “caught” the wave, they could create a little spark that he could see. He could then change the size of the gap to give him a sense of the strength of the wave.  [By the way, he started with a square receiver with a gap but found the position of the gap would change the size of the spark so he switched to a circular receiver so that he could adjust the position of the spark gap by rotating the receiver].

Heinrich Hertz lab

This was not an easy experiment, Hertz recalled that “the sparks are microscopically short, scarcely a hundredth of a millimeter long.  They only last about a millionth of a second.  It almost seems absurd and impossible that they should be visible; but in a perfectly dark room, they are visible to an eye, which has been well-rested in the dark.  Upon this thin thread hung the success of our undertaking.”  On November 5, 1887, he hesitantly sent a paper of his work to his old boss Helmholtz, “I have some misgivings about taking up your time but this paper deals with a topic that you yourself once urged me to tackle some years ago.”  Helmholz’s reply was immediate, a postcard that simply said, “Bravo!  Will hand it to be printed on Thursday.”

Suddenly Hertz was on a roll.  His wife Elisabeth wrote to Hertz’s parents that work was going excellently, “He simply pulls these beautiful things out of his sleeve now!  Of course, it makes him very happy, and me as well, when he tells me about it with a radiant face [although] I certainly understand nothing of it.”

Hertz then noticed that ultraviolet light altered his results, making him the first to notice the photoelectric effect. (I will revisit the photoelectric effect in video #38 when I get to Quantum Mechanics.)  Then, to Hertz’s surprise, he noticed that his new wave bounced off of mirrors, just like visible light does.  So, Hertz set up a mirror parallel to the spark so that the wave would bounce back and forth and make a standing wave.  Standing waves are distinguished by the fact that they have points where the waves oscillate strongly (antinodes) and points where the waves destroy each other (nodes).  Hertz moved his receiver between the antennae and the mirror and noted that the spark in his receiver would grow bigger at the antinodes and would disappear at the nodes.  In this way, Hertz measured the wavelength of the wave.  He also knew from the theory that the spark gap generator produces waves of frequency of around 70 million waves per second, or 70 million Hertz (named after Heinrich).  He then used the simple wave equation that speed is frequency times wavelength.  In this crude way, he measured that the speed of his wave was 320,000 km/s, which is quite close to the speed of light (300,000 km/s).  [Note: he didn’t break the speed of light, he just had small inaccuracies in his measurements].  Hertz had not only verified the idea that light was an electromagnetic wave, but he had also created his own world of invisible low frequency “light” that has transformed our world!

Heinrich Hertz discovering radio waves
Heinrich Hertz discovering radio waves

Hertz’s papers gained him almost instant international fame.  William Thompson, who had influenced both Faraday and Maxwell in their theories, said, “Hertz’s electrical papers are a permanent monument of the splendid consummation” of Faraday’s ideas that, “offended physical mathematicians” 56 years earlier.  Hertz became a professor at Bonn University and bought a beautiful house with a slight “catch” that it used to be a medical clinic that might have been contaminated by chemicals.  Maybe for that reason or maybe for some other cruel trick of fate, Hertz began to have migraine headaches in July of 1892.   He was diagnosed with a “blood disorder” and died on January 1st of 1894 when he was only 36 years old leaving his distraught wife Elisabeth and two young daughters, aged 2 and 6.

It is tempting to think that if it weren’t for his untimely death, Hertz would have invented wireless telegraphy and maybe even radio communications.  However, when he was alive, he saw no practical uses for his discoveries, saying that his experiments were, “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.”

A few months after Hertz’s death, a 20-year-old Irish-Italian man named Guglielmo Marconi read an obituary of Heinrich Hertz and became obsessed with creating long-distance wireless telegraphs.  However, he might not have gotten very far as the spark gap generator wasn’t powerful enough to transmit radio across an ocean.  Luckily for Marconi, Nikola Tesla had gone to the World’s Fair in Paris and heard about “the miracle” of Hertz’s experiments.  Tesla then started tinkering with the spark gap generator and ended up inventing the Tesla coil!  But what exactly is a Tesla coil, how does it work, and why is it important?  Well, I’ll tell you next time about the secret history of electricity. 

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