A century ago, the January 1917 issue of Popular Mechanics showed this surreptitious radio receiver having the size and appearance of a fountain pen. It was said to “make it possible for a man in the streets to pick up messages sent out from any station in the immediate vicinity.”

A tiny speaker was mounted at one end, behind which three small batteries provided power. The miniature audio tube was mounted at the other end, with coil and condenser behind it. Tuning was effected my moving a ring encircling the coil. Tiny connectors on each side were provided for antenna and ground. The antenna could consist of a wire running down a sleeve to a cane, and the ground was a wire running down the trouser to a metal plate in the heel of one shoe. To listen, the user could hold the cane at arm’s length into the air and put his heel against a fire plug or other metal object. By holding the tiny speaker into his ear, he could pick up messages without difficulty.