Component record · Regulation
Pendulum Bob
VerifiedThe weighted mass at the bottom of a pendulum rod, providing most of the pendulum's inertia.
What it does
Concentrates mass at the pendulum's lower end so the system swings with a regular period determined largely by the rod's effective length and the bob's position.
How it works
The bob is typically a disc or cylinder of dense material threaded onto the lower end of the rod, held at an adjustable height by a rating nut turning on a threaded extension of the rod.
Raising the bob shortens the pendulum's effective length and speeds the clock; lowering it lengthens the pendulum and slows the clock, giving a simple fine-adjustment method familiar to longcase clock owners.
In mercury-compensation pendulums, the bob itself is a jar of mercury whose level rises with temperature-driven expansion, moving the mercury's centre of mass upward to compensate for the simultaneous lengthening of the rod.
Evolution
Simple lead or brass bobs remain standard in domestic clocks; compensation bobs (mercury or gridiron) were reserved for precision regulators and observatory clocks.
Known variations
- Plain brass or lead disc bob
- Mercury-jar compensation bob
- Lenticular (lens-shaped) bob favoured in some fine clocks for reduced air resistance
Identification for collectors
- The weighted disc visible at the bottom of a pendulum, with a rating nut typically beneath it
Common failure modes
- Bob coming loose on the rod, disturbing beat and amplitude
- Mercury bob leakage in antique mercury-compensation pendulums, a hazard requiring specialist handling
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source 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.