Electronically-Driven Pendulum

Fused Quartz Rod, The Ideal Pendulum Shaft Material

Temperature compensator cancels residual thermal drift.

Max Carter

Fused quartz has virtually zero coefficient of thermal expansion (CTE), but the pendulum's metal suspension and bob components leave the suspension-rod-bob system with some residual positive CTE. Compensating components in the bob system provide adjustable negative CTE feedback. The result is a pendulum minimally affected by temperature change.


  • Overall pendulum length: ~42.5" (1080mm), including suspension and bob. Effective length can be adjusted by varying the height of the two cylindrical posts in the suspension. The original article explains.
  • 4.5mm rod is a snug fit in nominal 3/16" (4.75mm) I.D. aluminum tube. May require drilling tube to 3/16" (4.75mm).
  • The brass parts (finial, nuts and nipple) are standard lamp parts; 1/8IPS is the most common size, available (for example) here.
  • The brass nipple may require slight reaming on the end to fit over the 1/4" (across flats) centering/lock nuts.
  • Verify that the compensator (comprising finial, threaded nipple and brass nuts) is free-floating, ie, able to freely slide up and down.
  • Note that expansion or contraction of the compensator (due to temperature change) acts counter to the residual expansion or contraction of the rod and bob. In other words, as the rod and bob expand downward, the compensator expands upward, and vice versa.
  • 4.5mm fused quartz rod is available here.
  • 10-32 x 1/4 small pattern nuts available here.
  • 1cm magnet available here.
  • The suspension spring is from a defunct retractable tape measure.
  • See here for the background article for this project.

Suspension Spring and Rod Attachment

*Effective pendulum length can be adjusted by varying the height of the two cylindrical posts in the suspension. See original article.

Bob and Compensator

As adjusted.

  • The 3" threaded nipple and nuts provide adjustable temperature compensation. The configuration shown in the photo was arrived at after much tweaking using a home-built electronic phase comparator.

72-hr Sample


Electronic Pendulum Driver

Running 72-Hour Phase Record (Graph)

Electronic Phase Comparator