When electric clocks became widely available in the 20th century, enterprising clockmakers began making and selling electric mystery clocks. A Dutchman named Leendert Prins made these clocks possible in 1932 when he patented a device to move an advertisement around a clock face with no apparent mechanism. He patented a modified similar device for mystery clocks in 1941. Prins' device used four transparent discs, with the hour hand glued to one of the inner discs, and the minute hand glued to the other inner one. Electric mystery clocks were all the rage in 1950s United States. Various companies made them, including Tiffany, Rex Cole, Boots Boy and LeCoultre [source: Linz]. Most of these used the Prins device.
The most popular and famous of the electric mystery clocks was the Golden Hour made by the Jefferson Electric Co. of Illinois. It was marketed for its mystery and because the clear clock face would go with any décor. Radioactive radium paint was used on the glow-in-the-dark hands and the numbers [source: Schultz]. Jefferson sold other versions including the Golden Minute, Golden View, Golden Secret and Exciting Hour [source: Linz].
New Golden Hour clocks sold for about $25, making them affordable for ordinary people. In the second decade of the 21st century, they're prized as collectibles. You can find Jefferson mystery clocks at antique stores, clock stores or online auction sites for prices ranging from less than twice the original price to several hundred dollars. They were manufactured into the late 1980s, when the company was sold and the clock division closed [source: Russell].
The Haddon Clock Co. of Chicago made mystery clocks that looked much like the Jefferson clocks, but the Haddon clocks had hands that obviously moved. A gear was hidden in the metal ring that held the clock face, and a tiny wire on the minute hand fit into the gear. Gears attached to the minute hand drove the hour hand. The minute hand is obviously touching the rim, so the mystery was easier to solve. Other manufacturers including Mastercrafters and the English company Smith's.
In the 1980s, mystery clocks were sometimes made for special occasions or companies, such as a Faberge Mystery Clock made for the Franklin Mint, and a Lowenbrau mystery clock made by Lakeside, Ltd.
Early in the 21st century, some manufacturers such as Dior and Louis Vuitton began adapting mystery clock techniques for novelty wristwatches with transparent movements and hands that appear to float. Whether collecting curiosities or admiring expensive artworks, many people are still intrigued by mysterious timepieces.