Full Chisel Blog

November 17, 2008

What would Huygens do?

Filed under: Historical Material,Of Interest,Techniques,Uncategorized — Stephen Shepherd @ 7:23 am

After all he invented the pendulum, plate tectonics and wave theory, in the 17th century.  It mostly has to do with frequency and that is determine3d by the length of the pendulum and to some degree its mass.  More importantly where that mass is located.  The usual location for the weight is in the bob, near the swinging end of the pendulum shaft.  Now why would I bring this up?  Well it all has to do with the wall clock I made.  Remember the one I have to move the mechanism and dial down.

I had put the mechanism on the back board a while back in order to offer some protection to the works before I built the clock case.  While on the board, I could not slow it down enough to keep proper time, it ran too fast.  The wire extension did the trick, but upon ‘completion’, the works needed to be dropped, the extension removed and the bob adjusted all the way down.  Still running a bit fast I was in a quandary.

 So I called the Professor (of Mathematics and Physics) and posed the question of how to slow down the pendulum?  Could I add weight to the bob?  He paused, mentioned an oncoming malady effecting his head and grabbed a math book to check his thoughts.  Then a physics book, then an interesting solution.

He suggested an increase in the gravitational forces directly under the clock would do the trick and could be accomplished with a small amount of a neutron star material.  I asked him to send me some, he said yes but that I would have to pay the postage.  Well, we thought it was funny.

Adding mass to the bob will only slightly decrease the swing.  What is important is the location of the weight.  He said by reducing the weight of the SHAFT of the pendulum would slow it down.  That to me was counter-intuitive, then I thought about John and James Harrisons work with heat compensating pendulums in the early 18th century.  John Harrison eventually was rewarded for his resolution of the problem of determining longitude at sea by developing the first chronometer (actually two movements that correct each other).

Because part of the shaft is wood, I can remove some of its mass by gouging wood from the back side and I think I will add some lead shot to the bob, it is hollow on this movement.  I actually have about 5/8″ below the bottom of the pendulum and could add a smaller extension, but I like the placement of the bob where it is, so I am willing to play with the physics and math.  I should have paid more attention in school.



  1. Ah, the joys of the pendulum.

    How fast is it running?

    I’m betting 4 bits (and that’s a very hefty bet for me) that extending the length will be required. There’s probably not enough mass in the shaft to make the difference you need. Beware of too much extra mass in the bob putting more friction on the escapement. Which reminds me that I have a clock needing cleaning. After about 30 years, it refused to start again after I stopped it to reset DST last fall.

    Good luck with it. It’s a fine looking clock!

    Comment by Bob Easton — November 17, 2008 @ 11:49 am

  2. Bob,

    I am sure I removed a few grams of wood from the shaft, not much, but I did add a fat piece of wire to the inside of the bob for a little more weight. It appears to be running about 5 minutes fast in 24 hours before the alterations. I checked after two hours (after the alterations) and it is on time. I will have to check tomorrow.

    As for the additional weight, it is isolated from the escapement, so there is no additional weight on the movement, but a very good point you brought up.

    There is just something much more fascinating about a mechanical movement as opposed to the kind that kills electrons.


    Comment by Stephen Shepherd — November 17, 2008 @ 1:10 pm

  3. Half of physics (until you start getting quantum) is visualization. Removing material from the shaft makes sense if you view the shaft and the bob as a total unit. What you are trying to do is move the center of gravity to the lower end of the structure. I suspect that a ridged shaft and solid structure will make the timing more consistent so too much material removed might be bad.

    Thank you for posting this, I am planning a clock and your shared experiences have already made me alter my design.


    Comment by Bob Strawn — November 18, 2008 @ 9:52 am

  4. Bob,

    Glad the post was helpful for your future clock project. I did go ahead an ‘lighten’ the shaft and added a piece of heavy wire around the lower part of the inside of the bob. I bent it in a circle and had to cut out a bit where the shaft exits the bob. You can see the wire at the bottom and the tips at the top. And of course you can see where I lightened the shaft. Well it worked and the clock was about 5 minutes fast this morning. I made a minor adjustment and will again check it on the morrow.



    Comment by Stephen Shepherd — November 18, 2008 @ 2:55 pm

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