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The Encyclopedia of Horology

Clock type · Specialist

Anniversary / 400-Day (Torsion) Clocks

Verified

Clocks regulated by a rotating torsion pendulum (typically a disc with three or four ornamental balls) suspended on a thin, twisting suspension spring, allowing an unusually long running time — nominally about 400 days — from one winding.

Movement & mechanism

  • Torsion pendulum, oscillating by twisting rather than swinging, connected to the escapement via a suspension spring rather than a conventional pivoted pendulum
  • Very low power consumption per oscillation permits the long (nominally 400-day) running period
  • Often displayed under a glass dome, with the rotating balls visible as a decorative as well as functional feature

Identification

  • Rotating disc with suspended balls beneath the movement is the defining visual feature
  • Glass dome case is near-universal for this type
  • Rate adjustment is typically made by raising or lowering small nuts on the suspended balls rather than a conventional pendulum-length adjustment

Terminology

  • Torsion pendulum — a pendulum that oscillates by twisting about a vertical axis rather than swinging
  • Suspension spring — the thin flat spring from which the torsion pendulum is suspended
  • 400-day clock — a common trade name referring to the nominal (rather than guaranteed) running time

Known variations

  • Four-ball torsion clock
  • Three-ball torsion clock
  • Skeletonised anniversary clock

Collecting notes

Anniversary clocks are widely collected, especially German and Austrian examples from the early twentieth century; the suspension spring is fragile and its condition/originality significantly affects both timekeeping and value.

Photographs

Haller torsion pendulum anniversary clock
Haller torsion pendulum anniversary clock — Graham Evans, Battersea Clocks Home (aka westminsterclocks), CC BY-SA 2.0. Wikimedia Commons file page
Anniversary Clock, made by Ferguson's Watch and Clock Shop, Walpole MA, undated - Bennington Museum - Bennington, VT - DSC08665
Anniversary Clock, made by Ferguson's Watch and Clock Shop, Walpole MA, undated - Bennington Museum - Bennington, VT - DSC08665 — Daderot, Public domain. Wikimedia Commons file page

Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.

Sources & references

  1. General horological reference literature on torsion pendulum clocks.

Where sources disagree, the disagreement is stated rather than resolved silently.