Component record · Regulation

Pendulum Rod

Verified

The rigid or compensating rod suspending the pendulum bob, whose effective length governs the clock's rate.

What it does

Provides the swinging arm of the pendulum, connecting the suspension spring at its top to the bob at its bottom, and largely determines the period of oscillation.

How it works

The pendulum's period depends on its effective length (essentially the distance from the suspension point to the bob's centre of oscillation), so the rod's length, and any change in that length with temperature, directly affects rate.

A plain steel or brass rod expands and contracts with temperature, lengthening in heat and shortening in cold, which speeds and slows the clock respectively unless compensated.

Compensating rod designs (such as the gridiron pendulum, alternating rods of different metals with opposing expansion, or later invar rods exploiting a low-expansion alloy) counteract this thermal effect to hold rate more constant across temperature changes.

Evolution

Plain rods were adequate for domestic clocks, but precision regulators and observatory clocks drove development of compensating rods through the 18th and 19th centuries, culminating in low-expansion alloys such as invar in the early 20th century.

Known variations

Plain wood or steel rod
Gridiron compensation pendulum
Mercury compensation pendulum (using mercury-filled jars as the bob to compensate thermally)
Invar rod (low thermal expansion)

Identification for collectors

Visible hanging inside a longcase or precision clock's case, connecting the suspension spring to the bob

Common failure modes

Bent rod from mishandling, affecting beat and rate
Corrosion or wear at the bob attachment point

Ellicott pendulum — Augustin-Privat Deschanel, Public domain. Wikimedia Commons file page

Sources & references

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

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