Location:  This 'hidden gem' is located at the intersection of 13th and K NW, in Washington, DC (see Clue Me! Map).  On a nice Spring day in 1880, Alexander Graham Bell made the first wireless phone call, once again demonstrating that he was an outside-the-box and ahead-of-his-time kind of guy. This was only four years after he invented the wired telephone, and was 13 years before Tesla invented the radio. It took nearly 100 additional years before a well functioning cordless telephone became available.

So, how did this work? He put a mirror near a megaphone, and positioned the mirror so sunlight hit at a slight angle and reflected onto the receiver a couple blocks away. He speaks into the megaphone. The sound makes the mirror vibrate. The receiver notices that the light wavelength compresses and decompresses, and converts that fluctuation into sound. Boom, the wireless telephone is born. Making it mobile ... well, that took a little longer.

This is all nicely demonstrated through a nifty DIY example by Dean Segovis. He shows how it works, and in a way that you too can construct your own photophone ... assuming you have access to a chunk of solar panel and some electronics gizmos. 

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Here's the hidden gem entry from our Clue Me! map.

Clue

The sunny roof of a neighborhood school

Description

The Franklin School, the roof in particular

Why It's Interesting

Alexander Graham Bell made the first wireless phone call from this building in 1880. He used a beam of sunlight to transmit a message to his lab a block north, where the receiving mirror vibrated and converted the message back to audio. While not practical (since no light was bright enough in those days other than the sun) the technology eventually set the stage for fiber optic communications we use today.

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