Crystals are key to a lot of radio designs. They act as a stable frequency source and ensure you’re listening to (or transmitting on) exactly the right bit of the radio spectrum. [Q26] decided to use ...

Marked: "Martian / Big Four / Newark, New Jersey / Pat. Pend.". A crystal radio detector with binding posts for headphone connections on top, posts for antenna and ground connections on bottom.

Marked: "Radiogem / The Radiogem / Corporation / New York". A kit to make a crystal radio receiver, including original package and instruction manual. Unit is a commercial variety of the old Quaker ...