Expert: Martin Meyer - 1/7/2010

Question
I have an antique grandfather clock with a mercury pendulum that is running approximately 15 - 20 minutes fast.  I have added mercury to the glass tubes and cannot add any more.  I have adjusted the weight on the pendulum located just above the mercury tubes to its lowest position.  I don't see any other mechanism by which to slow the clock down.  Am I missing something? Any insight would be greatly appreciated.

Answer
Hi Michele,

The weight of the pendulum is unrelated to the rate of the clock, except in a very indirect way. Additionally, mercury is highly hazardous and should not be used.

Mercury pendulums often have a nut above the mercury tube unit which is used to keep that unit fixed and in position. Regulation is usually accomplished the standard way, by lowering the mercury weight unit on the pendulum rod. The longer the pendulum, the slower the clock will run, and the lower the weight is, the longer the effective length of the pendulum is. Your addition of the extra mercury has raised the center of mass of the pendulum, thus making the pendulum effectively shorter, and making the clock run faster. I would not recommend  removing the extra mercury yourself unless you are qualified in handling hazardous materials.

The clock may have a regulating hole in the dial. This is a small "S-F" regulating hole in the dial, usually near the 12. You turn the shaft inside this hole in small increments with the small end of the winding key. If your clock does not have this feature, it should have a regulating nut for lowering the pendulum. This is not the nut above the mercury tubes. The regulating nut is probably either at the bottom of the mercury tube assembly, or is sometimes a circular disk-type nut in the area of the mercury tubes. Some clocks, especially ones with the small "S-F" regulating hole, will have a set screw in the back of the pendulum, which can be used to adjust the mercury tube assembly height.  http://www.clockmasteronline.com