What Is A Ball Clock?
The clock described in the ACM programming problem has three tracks that hold ball bearings. Once a minute, a rotary arm lifts a ball up and drops it in a channel where it rolls onto the topmost track. When five balls have accumulated in the top track, the weight of the balls tips the track down. One ball rolls down to the middle track, and the rest roll to the pickup queue at the bottom of the clock. The middle track thus has a ball for every five minutes of time. When the twelfth ball rolls onto the middle track, it dumps one ball onto the bottom track and sends the rest to the pickup queue. So the bottom track has a ball for each hour. Each track is labeled with hours or minutes. To read the time, you look at the bottom track and read off the hour, then you add the minutes shown on the top two tracks.
Public Clocks
George Rhoads has created a number of rolling ball sculptures (some of which may be clocks) that are installed in public places. For more information, visit George's web site, or see the article "Clumper Upper to Wok Dumper to Chute to Helix to Block" in the October 1988 issue of Smithsonian magazine. Here's an excerpt: The paths taken by George Rhoads' flashing little sound-making balls are varied and magical, making art of hypnotic movement. They've been showing up all over North America these past few years -- these intriguing machines with acrobatic balls that move around metal tracks and make noises as they go. To the man who made them, artist George Rhoads, they are "audiokinetic sculptures" that generate sounds as they move.