Eighty years ago,  Aeolian’s store in St. Louis (apparently a dealer in radios and pianos) managed to attract a big crowd to the sidewalk in front of their store window with this clever display, shown here in the October 1937 issue of Radio Retailing. The plate glass window contained four “buttons,” consisting of cards mounted inside the glass, each with the call letters of a radio station and the words “Place Hand Here to Tune.” Sure enough, when a visitor placed his or her hand on the glass, the console radio four feet away mysteriously tuned to the selected station, and the program came over a sound system piping the radio to the street. Over the course of three days, 5000 people placed their hands over the cards to tune in the radio. Many of them went into the store for a better look.

According to the magazine, the display was the brainchild of parts jobber Bob Ferree of Interstate Supply and Fred Pitzer of RCA. The cost of materials was $50, which the magazine proclaimed to be “cheap, when the timliness and effectiveness of the stunt is considered.”

The magazine carried the full schematic for the circuit, shown below. As most readers probably guessed, the circuit relied upon hand capacitance. Each foil pad was connected to an oscillator circuit, set so that the added hand capacitance would stop the oscillation. This triggered a relay, which in turn triggered a second latching relay which tuned the set’s remote control to the selected station.

Each of the cards had behind it 3.5 by 5 inch strips of tinfoil. Another card was glued to the back to conceal the foil. From each piece of foil ran a tiny 40-gauge enameled wire which ran to a relay unit hidden behind the set.  Each station required two tubes plus two relays, plus one rectifier tube to power the entire unit.