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The Encyclopedia of Horology

Clock type · Precision

Free-Pendulum Precision Clocks

Partially verified

Clocks in which the timekeeping pendulum is left almost undisturbed while a second clock performs the work.

Movement & mechanism

  • A free pendulum swings in a low-pressure enclosure and receives impulses only intermittently
  • A slave clock performs the counting and drives the dials
  • Temperature and pressure are controlled to reduce rate variation

Identification

  • Two linked clocks, one of which does no display work
  • A vacuum or low-pressure pendulum enclosure
  • Surviving examples are largely institutional

Terminology

  • Free pendulum
  • Slave clock
  • Invar pendulum rod

Known variations

  • Shortt-Synchronome and related systems

Collecting notes

Originality decides value and interest: check that movement, dial and case belong together, and prefer honest wear over refinishing. Movement work belongs with a clockmaker; cleaning a dial or case yourself commonly destroys the evidence that dates the clock.

Diagram

escape wheelpallets and leveroscillator
Schematic: escapement arrangement. An outline schematic drawn to show the arrangement of the parts discussed here. It is not a dimensioned working drawing and does not depict a particular maker's calibre.

Sources & references

  1. Britten, F. J., Old Clocks and Watches and Their Makers
  2. Loomes, B., reference works on British clockmakers (edition-dependent)
  3. Museum catalogue descriptions of comparable examples

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