An Indelible Memory –

For those of us old enough to remember the Steve Miller Band, the remaining words to the verse “Time Keeps on Slippin’” are so indelibly etched in our minds that they almost appear as text along with the accompanying music and easily recognized synthesizer “space intro” to the song “Fly Like an Eagle.”  Memories are like that, a small suggestion is usually all that is required to let your mind compose a picture of the past, often complete with audio.  While music is a powerful trigger of that memory recall process, images often do the same thing.  The torsion pendulum clock, also known as the anniversary clock or 400 day clock, is one of those images to many people.  If you don’t remember the name, you will surely remember someone, maybe an aunt, uncle, or grandparent, that had one in their house (usually on a mantle or shelf).  The anniversary clock was a glass domed mechanical clock that had 3 or 4 pendulum balls at the bottom that rotated back and forth.  Once you have seen this unique clock, you will not forget it.

The Torsion Pendulum

A torsion pendulum or torsional pendulum is a mechanical oscillator, typically a disc or similar mass suspended from a torsion wire.  The torsion wire does not extend along its length, but is free to twist or rotate about its longitudinal axis.   As the torsion wire twists, the suspended mass will in turn rotate along a horizontal plane that is perpendicular to the longitudinal axis of the torsion wire.  The suspended mass is imparted with a torque to start the pendulum, and as the torsion wire twists, it begins to resist further deformation and develops a restoring torque that seeks to restore the torsion wire to an untwisted state.  A torsion wire has a torque constant similar to the spring constant defined in Hooke’s law.  So the action of a torsion pendulum can be expressed as a simple harmonic equation. When a torsion pendulum is imparted with movement (such as from a clock mechanism), it will oscillate torsionally at a fixed frequency that is dependent on the torque constant of the wire and the moment of inertia of the suspended mass.

…And the Clock Behind It

So enter the torsion pendulum clock.  Based on the properties of a torsion pendulum, the torsion pendulum clock or anniversary clock keeps time based on the predictable properties of the torsion pendulum.  This type of mechanical clock has a long slender torsion wire with three or four balls attached to a spoke like arrangement attached to the wire.  The suspended balls rotate clockwise and then counterclockwise, a type of twisting rather than swinging pendulum.  The clock is spring wound, and a series of gears imparts a periodic torque to the top of the torsion wire to keep the wire and suspended balls in oscillation. This very predictable oscillation controls the movement of the clocks hour and minute hands.  Torsion pendulum clocks use very little energy to keep the wire and mass oscillating, and came to be known as “400 day clocks,” as many can run for more than a year on a single winding.  The term “anniversary clock” also came into common use, as clocks were often given as gifts to commemorate an important occasion such as an anniversary or a birthday.  The clock owner would be reminded to wind the clock simply by remembering the special occasion.

 A Short History of the Clock

The Torsion Pendulum Clock was invented and patented by American Aaron Crane in 1841. While torsion pendulum clocks appeared as early as 1841, they began their popularity when the German Anton Harder also invented the torsion pendulum clock in 1879, allegedly independent of inventor Crane.  Production of the torsion pendulum clock started around 1881 in the Black Forest region of Germany.  August Schatz founded the Wintermantel company there, which later became the Jahresuhrenfabrik company.  This company licensed a patent granted to Harder, but in 1887 the patent was left to expire, and various German companies soon began producing the torsion pendulum clock.  Companies such as Kieninger & Obergfell (Kundo), Schatz,  Gustav Becker, Lenzkirch and Kienzle & Junghans all produced torsion pendulum clocks housed under a glass dome.  The industry grew and expanded greatly.  They proliferated in America after World War II when returning American servicemen acquired them as souvenirs from their time in Europe.  The clocks were an exquisite example of mechanical engineering, and were also delicate and did not always keep good time.  The torsion spring, for example, was sensitive to temperature variations.  These 400 day clocks therefore needed adjustment and maintenance, and the growth of these clocks also saw the growth of clock shops to service and adjust them. The suspended balls, for example, can be adjusted inward or outward to adjust the time keeping of the clock. Like most inventions, improvements came with time.  Charles Terwilliger invented a temperature compensating torsion wire that further advanced the technology of this most unusual time keeping instrument.

Where are They Today?

Anniversary clocks can still be found in antique shops, estate sales, and of course eBay.  They will also turn up occasionally on Craigslist.  They can be found in various states of repair, and are considered a collectible.  There are still some clock shops and enthusiasts that specialize in repairing and selling anniversary clocks, so a small yet lively market still exists for these clocks. They are a thing of engineering beauty worthy of display.   If you are lucky enough to come across one of these clocks, enjoy the time lost beauty of their mechanicalness – batteries or charging cable not required.

GRAPHIC  CREDIT:  “KUNDO Anniversary Clock Needing Repair”. Copyright 2018.  Robert Gunderman.

Authors Robert D. Gunderman P.E. (Patent Technologies, LLC www.patentechnologies.com) and John M. Hammond P.E. (Patent Innovations, LLC www.patent-innovations.com) are both registered patent agents and licensed professional engineers.  Copyright 2018 Robert Gunderman, Jr. and John Hammond

Note:  This short article is intended only to provide cursory background information, and is not intended to be legal advice.  No client relationship with the authors is in any way established by this article.