Pendulum Clocks

Movement & mechanism

• A weighted bob suspended on a rod or wire, swinging with a period governed chiefly by its effective length
• Coupled to the going train via an escapement (verge, anchor or deadbeat) that gives it small periodic impulses while counting its swings
• Longer 'seconds' pendulums (about one metre) beat once per second and are typical of longcase and regulator work; shorter pendulums suit wall and mantel clocks

Identification

• Visible swinging pendulum, either enclosed (longcase, many wall and mantel clocks) or exposed (some Comtoise, Vienna regulator and skeleton clocks)
• Pendulum length correlates roughly with beat rate: a full seconds pendulum is markedly longer than a half-seconds pendulum
• A rating nut beneath the bob allows fine adjustment of the effective pendulum length, and so the rate

Terminology

• Bob — the weighted mass at the base of the pendulum
• Rating nut — the adjustable nut used to raise or lower the bob to correct the rate
• Isochronism — the ideal, only approximate, property of a pendulum's swing period being independent of its amplitude

Known variations

• Seconds (long) pendulum clocks
• Half-seconds and shorter pendulum clocks
• Compensated pendulums (e.g. gridiron, mercury) in precision regulators

Collecting notes

As a mechanism category, pendulum clocks are collected under their specific forms; however, the escapement used to control the pendulum (verge, anchor, deadbeat, gravity) is itself a significant dating and grading criterion across nearly all pendulum clock types.

Sources & references

1. Britten, F. J., Old Clocks and Watches and Their Makers

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