Pendulum clocks can be incredibly consistent if they are in a stable environment. My dad built a grandfather clock with a Hermle movement. With nothing more than fine adjustments to the pendulum length (using a convenient screw for just this purpose), it can be tweaked to within about 3 seconds per week. It is temperature sensitive, needs to be adjusted each spring & fall.
There were strategies for that, too. One was to use a glass vial filled with mercury as the weight—the shape of the vial could be designed so that as the mercury expanded, it would counteract the effect of expansion on the pendulum stick. Another was to alternate brass and steel rods arranged to offset thermal expansion. Then came the use of Invar, a nickel alloy, that has a near-zero thermal coefficient. Better watches use Glucydur for the balance wheel for that purpose.
The best clocks used a very heavy weight and the smallest arc possible to avoid circular error (the gravity vector is only at unity when the pendulum stick is vertical). This required superb craft on the escapement and verge—the smaller the movement of the pendulum, the greater the effect of a given machining error.
The period is controlled by the length. Tall case clocks are tall because a one-meter pendulum beats 3600 times an hour (once per second), which is convenient.
I have a tall case clock that I built with a standard modern (cheap) movement. It’s not too bad—within a minute a week. The pendulum’s arc is large with the empty-shell bob. I had to limit the size of the bob to keep it from banging the sides of the case.
But I have a 120-year-old Gilbert jeweler’s regulator that has a wax-impregnated wooden pendulum stick, a dead-beat escapement, and a 4” bob that is filled with lead and heavy. The swing is about two inches with a period of about 2/3 second. I can regulate that one to a handful of seconds per week. It was used as a reference clock at a jewelry store for regulating customer clocks.
Both are weight-driven and thus maintain good isochronism. A spring-driven clock had to be used on ships, and those used a funnel-shaped spring barrel called a fusee to counteract the decline in force as the spring winds down. For stationary clocks, weights are easier.
I have a couple of dozen clocks in the house, but I don’t usually run them. The ticking drives the Redhead mad (I’m already there) and you’re toast if in the middle of a Zoom meeting at noon, especially towards the end of the week when the 8-day clocks would be drifting apart. Other than the tall case clock, the newest one dated from the 1920’s.
Rick “watches take less space” Denney
