The seventh entry in the Top Eleven is an experiment that leads directly to all forms of wireless communications.
Who: Heinrich Rudolf Hertz (1857-1894), a German physicist.
What: Hertz studied electromagnetism, and in particular, the prediction from Maxwell's Equations that it ought to be possible for electromagnetic waves to travel through free space. Today, this is a simple demonstration-- you grab a couple of loops of wire, a signal generator, and an oscilloscope, but in 1886, it required the invention of a good deal of apparatus.
In order to show that electromagnetic waves travel through space, you need two things: a method of generating oscillating fields, and a method of detecting them some distance away. Hertz's solution to these problems was both ingenious and kind of unweildy: he made an oscillating circuit containing a big capacitor, a coil of wire, and a spark gap. When he charged the capacitor up and connected it to the gap, the voltage across the gap would increase until the breakdown point was reached, at which point, a spark would jump across the gap, and an oscillating current would flow (through the coil of wire) for a short time. This produced a large oscillating field, and thus waves that could be detected.
Hertz used the same basic principle to make his detector. He set up a loop of wire with an adjustable gap in it, and set it some distance away from the generating spark gap. When the resonant frequencies of the detector and generator were tuned to match, and the detector was aligned properly, he could see a spark jump across the gap in the detector, indicating that the waves had traveled across the intervening distance.
(Wikipedia is a big let-down on this subject. You need to turn elsewhere on the Web to find a good description of Hertz's experiment, a more modern implementation, and pictures of his apparatus, which is nifty in that polished-wood-and-brass way of so much 19th Century scientific apparatus.)
Why It's Important: This was the first convincing demonstration of the generation and detection of electromagnetic waves. Hertz also managed to prove that the waves were polarized, that they could be reflected and refracted, and that they moved at the speed of light, conclusively demonstrating that light is a form of electromagnetic radiation.
In the bigger picture, Hertz's experiments led directly to the work of Marconi and others in developing radio, which in turn led to television, satellite communications, cell phones, wireless Internet, and pretty much anything that gets an electrical signal from one place to another without having wires connecting them.
As a nice bonus, he incidentally discovered the photoelectric effect while investigating the behavior of his spark-gap detectors. Ironically, the photoelectric effect, discovered in the course of experiments to prove that light is a wave, would start a long process of discovery that would prove that light actually behaves like a particle...
Reasons to Vote for Him:: His experiments are pretty much the culmination of a century or two of classical electromagnetism. His work led to radio, television, wireless Internet.
Reasons to Vote Against Him: His apparatus was pretty difficult to use, and the technique never went much farther (see excerpts of this book). His work led to annoying people talking on cell phones in public places.
I have to vote for Hertz. As mentioned the equipment for the experiment had to be invented from whole cloth and had to be "tuned" for something he didn't really know existed. It was couter-intuitive that something could jump through empty spce. Also, it was designed to demonstrate something that was only know from an abstract set of equations. Opening up the electromagnetic spectrum to be produced and studied was no small feat as well.