First trans-Atlantic radio transmission
On this day in 1901, Italian inventor Guglielmo Marconi made an important experiment in a whole series of experiments to make his wireless communication device practical.
Ever since 1893, Nikola Tesla and 
Marconi had (separately) been inventing and reinventing the radio, but the devices, which were called “wireless telegraph,” still could not send music and speech. Instead, they sent signals such as Morse Code. Some people said that radio would never be able to compete with telegraphy (which involves Morse Code signals sent along wires) because radio waves could only be transmitted along line-of-sight and at limited distances.
That’s one reason the December 12th experiment was important; Marconi showed that a radio transmission from a high-power station could be detected as far away as 3,500 km (2,200 miles).
It would be another five years before the first true AM radio messages—speech and music!—would be made.
What is radio?
Radio waves are just like light, but with slower frequency and much longer wavelengths.
A radio is basically an antenna to catch radio waves, some electronics to turn the waves back into sounds, and a loudspeaker so we can hear the sounds.
Einstein explained radio this way:
“You see, wire telegraph is a kind of very, very long cat. You pull his tail in New York, and his head is meowing in Los Angeles. Do you understand this? And radio operates in the same way: You send signals here; they receive them there. The only difference is that there is no cat.”
Here is a nice explanation of how radio works.
And here is a 1937 short movie on the same topic.
Are radios still being invented and reinvented?
Remember, invention goes on even for devices that work pretty well, because people can think of ways to make things smaller / bigger / faster / cheaper, and so forth. Right now the biggest thing in radio is probably satellite radio, which is like satellite TV. Sirius users in the U.S. can get their favorite sat radio stations everywhere they go, as long as they have a clear view of the satellites. They get a choice of more than 150 stations, and many of the stations are commercial-free. Users do have to pay a subscription for the service, of course (again, like satellite TV).
Kids’ Music on Kids’ Radio
Find a station near you here.
Cosmic Radio 
Did you know that stars, pulsars, nebulae, and galaxies emit radio waves?
Our huge, dishlike radio antennae not only collect our own radio waves from satellites, but also collect natural, random-sounding hisses from all over the sky. We can learn a lot about astronomical objects using an array of radio telescopes.
Here’s something weird to think about:
We have been sending radio and television transmissions out into space for more than 100 years. So it is possible that any aliens who live within 100 light years of is have detected those signals and are, perhaps, learning our languages, enjoying our old shows (do aliens love Lucy, too?), or shrugging in bewilderment over our rock-n-roll.
What would aliens think about humans from listening to and watching our current radio and TV?