Wood Rod and Lead Bob
American Watchmaker and Jeweler (1892)
A cheap and good compensating pendulum may be made with a wood rod and lead bob. For a seconds pendulum the rod should be .5 inch in diameter, of thoroughly well-seasoned straight-grained pine 45 inches long, measuring from the top of the free part of the suspension spring to the bottom of the bob. A slit for the suspension spring is cut in a brass cap fitting over the top of the rod, to which it is secured by two pins. A bit of thin brass tube is fitted to the rod where it is embraced by the crutch. The rating screw, .25 inch in diameter, is soldered to a piece of brass tubing fitting over the rod and secured by a couple of pins. Wooden rods require to be coated with something to render them impervious to the atmosphere. They are generally varnished or polished, but painting them answers the purpose well. Mr. Latimer Clark recommends saturating them with melted paraffin, The bob, 2.25 inches in diameter and 12 inches high, with a hole just large enough to go freely over the wood rod, rests on a washer above the rating nut. Shorter pendulums for chime and other clocks are made of cherry, mahogany, and ebony, simply because in such small sizes pine does not allow of sound attachment to the ends. These pendulums have generally lenticular-shaped bobs. Such rods cost scarcely any more than brass or iron, and are infinitely preferable. It is essential that the grain of a wood pendulum rod should be perfectly straight, for if the grain is not straight, the rod is likely to bend, causing the clock to go very irregularly.
What Has Changed in the Last Century
When the American Watchmaker and Jeweler gave these instructions in 1892, a varnished pine rod with a lead cylinder was the cheap way to cancel out temperature error, and the writer thought such rods "infinitely preferable" to brass or iron. Within a few years the alloy Invar, developed by Charles Édouard Guillaume in the 1890s and honoured with the 1920 Nobel Prize in Physics, gave clockmakers a metal that barely expands at all, and the finest regulators went to Invar rods in vacuum tanks rather than seasoned wood. Precision timekeeping then left the pendulum altogether: quartz oscillators in the late 1920s, and from 1967 the second defined by the caesium atom. Wooden rods are now the province of antique-clock restorers, who still follow much of this advice — including the warning that crooked grain makes a clock keep bad time.
Written for this edition, 2026.