3 Penny Radio

In science the Penny takes more abuse than any other denomination of American currency. A century ago Herbert Willis famously bombarded pennies with radiation just for giggles... and testing alpha rays. They were the currency of tchotchkes: penny candy, penny post cards, penny arcades and so on. We crush them into oblate shapes in hand cranked machines at truck stops to imprint them with hearts and images of tourist attractions.

The ultimate indignity was in radio, we soldered them. In his autobiography, Steve Wozniak mentions the archetypal use of the penny as the "crystal" in the crystal radio. In that application it's more accurate to say that the cuprous oxide on it's surface is the detector but it's still a penny. It's a use that dates back to at least WWII in early foxhole radios that used American Pennies, German Pfennigs, and old copper French centimes. The 2003 book Sneaky Uses for Everyday Things by Cy Tymony revisits that old design.

But getting back to the foxhole radios, the most common crystal in use by consumers around WWI was probably galena. Metallic detectors were known to exist. Soldiers innovated one that used razor blades which we call the foxhole radio. Iron(III) oxide, aka rust, is a semi conductor. This was discovered by Greenleaf Pickard in 1906. He also patented the  silicon crystal detector that year. It's likely that few, if any men in the trenches were aware of his discovery, more likely that they discovered it independently through trial and error. They already knew that galena could be used... someone probably had the sense to experiment with other metals. It wasn't until 1927 that Lars Olai Grondahl and Paul H. Geiger invented the copper oxide rectifier. More here. they both worked at the Union Switch & Signal Co.  While investigating the corrosion of copper switches they discovered that current flowed more easily one way than the other and made the crucial deduction.

That radio discovery has a modern descendant in the 3-Penny Radio. In this application the Penny is not the detector. The Pennies don't do anything electrically significant in this design. But that's OK. Projects like this are for kids, and half of the adventure is really just soldering practice. It's based on older designs, lacking even an oscillator. But it concedes to kid friendliness with an integrated circuit to handle AGC (automatic gain control) so that when junior hooks up the earphone the results will be more listenable. AM is so much noisier now than it was a century ago AGC is necessary to make the radio experience at all compelling.