The Rolling Ball Web
An Online Compendium of Rolling Ball Sculptures, Clocks, Etc.
By David M. MacMillan et. al.
Rolling balls can function in at least three different modes in timepieces: they can be involved in the keeping of the timebase itself, they can display the time, and they can be a part of the clock's power system. All of the rolling ball clocks with which I am familiar employ balls in only one of these major categories, but nevertheless these categories are not necessarily exclusive.
In order to understand how rolling balls can function in the timekeeping system of a timepiece, it is necessary first to understand the way in which most timepieces use oscillators.
Timepieces such as sandglasses, candles, fire clocks, and most water clocks (clepsydrae) do not use oscillators. They represent the flow of time by the continuous flow of some substance (sand through a glass, water through a spout, the consumption of incense), measuring this flow and correlating it to the time. Sundials and nocturnals measure time directly, acting as instruments to show the position of the sun and stars. With the exception of sundials and nocturnals (which, if correctly made and used, are as accurate as the motions of the heavens themselves), continuous timepieces have never been sufficiently accurate. It is very difficult, for example, to ensure that water flowing in or out of a clepsydra is flowing at a constant rate as temperature, atmospheric pressure, the consistency of the water, and the condition of the clock all change.
Insofar as rolling balls in quantity can simulate some of the properties of a fluid, it would be possible in theory to build a rolling ball continuous flow clock. Indeed, a sandglass approaches this, and a sandglass filled with perfectly spherical small beads would be a sort of a rolling ball clock. However, I know of no instance of a rolling ball continuous clock using macroscopic balls.
Since the invention of the mechanical escapement in the 13th century, most timepieces have measured time not by measuring a continuously flowing analog to time but instead by counting fixed intervals into which time has been chopped. It turns out that it is much easier - and more accurate - to generate a very regular short event, such as the swing of a pendulum, than it is to ensure the regularity of an ongoing event such as the flow of water. Early clocks employed a bar which swung back and forth, usually in a horizontal plane. This bar, called a foliot, was pushed in one direction by the "verge" escapement. At the end of its travel in that direction, it was caught by the verge escapement and pushed back in the other direction; at the end of this travel, the verge escapement again reversed its motion, and the cycle continued.
The foliot, or as it is more commonly termed, the "verge and foliot" (not to be confused with the Virgin Folio) was historically a very successful design. It continued in use for several centuries. However, it suffered from the disadvantage that although it was an oscillator (it moved back and forth), it had no natural period of oscillation. If supplied with more power, it would oscillate faster, if supplied with less, slower. It was thus very dependent upon the evenness of its driving power - and this power was typically not very even.
A number of improvements were made in the late 16th and early 17th centuries to the verge and foliot. Some of these, such as the use of bristles to help limit the motion of the foliot, and Jöst Burgi's cross-beat escapement were important developments contemporary with early rolling ball clocks. However, it was only with the introduction of the pendulum by Christiaan Huygens in the 17th century that an oscillator with a natural period - a resonator - was introduced into timekeeping.
Other oscillators and resonators have been used since then. For instance, the balance wheel and hairspring combination common in mechanical watches is a resonator, as is the quartz crystal of a modern wristwatch or the caesium atoms used in an atomic clock.
In developing a taxonomy for rolling ball clocks, then, we can first divide the major category of rolling ball timekeeping on the basis of two criteria: oscillating vs continuous, and direct participation in the timekeeping process vs. assisting that process. Logically, this gives four categories:
Category 1 has many important representatives, and may be further subdivided into devices where the rolling ball path is fixed (and the balls traverse it in one direction), devices where the rolling ball path moves (as in the Congreve), and devices in which a ball oscillates back and forth along a stationary path.
Category 2 has fewer representatives, but they are all of considerable interest. Because of the ability of a rolling ball to decouple motion, rolling balls have proven useful in transmitting motion to otherwise "free" pendulums in precision pendulum clocks.
Category 3 has, I believe, no representatives other than, after a fashion, the sand-glass. Still, it is possible to conceive of very interesting timepieces in this category.
I cannot, however, conceive of a timepiece in Category 4. This category appears to be of theoretical interest only - though I would be happy to be shown wrong on this point.
This category is not as frivolous as it sounds, at least from the point of view of cultural history. During the early 17th century rolling ball clocks were experimented with at some of the more prestigious scientific courts of Europe. The rolling ball clock therefore became an item with which one could signify one's techological prestige. However, the clockmaker to such a minor court might not actually understand what the rolling ball was for. von Bertele has identified at least one clock in which the rolling ball is entirely decorative.
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