Heinrich Hertz was a brilliant German physicist and experimentalist who demonstrated that the electromagnetic waves predicted by James Clerk Maxwell actually exist. Hertz is also the man whose peers honored by attaching his name to the unit of frequency; a cycle per second is one hertz.
The usual path of science is to go from phenomenon to theory. For instance, Darwin tried to make sense out of the interrelationships he observed between species and thereby gave us the theory of evolution and natural selection. But occasionally, events run the other way. The amount of energy condensed into matter was inconceivable before Einstein's little equation, e = mc2, and the atom bomb was proof of principle. Radio-waves pre-existed Maxwell's theory, published in 1865, but nobody would have known to look for them.
Maxwell's equations united the fields of electricity and magnetism and comprised the first field theory in physics. "It is impossible to study this wonderful theory without feeling as if the mathematical equations had an independent life and intelligence of their own, as if they were wiser than ourselves, indeed wiser than their discoverer, as if they gave forth more than he put into them," said Hertz. Of course they gave only to those that had the ability to interpret them, and fortunately Hertz was a pretty fair mathematician. As he also observed, "There are many lovers of science who are curious as to the nature of light and are interested in simple experiments, but to whom Maxwell's theory is nevertheless a seven-sealed book."
Hertz used a simple homemade experimental apparatus, involving an induction coil and a Leyden jar (the original capacitor) to create electromagnetic waves and a spark gap between two brass spheres to detect them. The gaps were difficult to see, and required that the he perform his investigations in a darkened room.
"For the sparks are microscopically short, scarcely a hundredth of a millimeter; they last only 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 hangs the success of our undertaking," said Hertz.
In later experiments, he was able to calculate the speed of the radio waves he created, and found it to be the same as the speed of light. A great number of subsequent developments, like radio and television, not to mention Wi-Fi, were spun out of his simple demonstrations. Hertz was well aware of the extent of his contribution. "We perceive electricity in a thousand places where we had no proof of its existence before. In every flame, in every luminous particle, we see an electric process. Even if a body is not luminous, provided it radiates heat, it is a center of electric disturbances. Thus the domain of electricity extends over the whole of nature."
[The Hertz quotes are taken from On the Relations Between Light and Electricity, a lecture delivered to German Association for the Advancement of Natural Science and Medicine, in 1889, in Heidelberg.]