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    Re: Leap seconds at Big Ben.
    From: Bill Morris
    Date: 2009 Jan 4, 13:54 -0800

    Richard Pisko asks:
    "Where is the pendulum weight fastened to the invar rod; and how?
    I was wondering . . .  if the rod were to have male threads at a certain
    pitch, say 20 threads per inch on a 1/2 inch diameter rod, and the brass
    cylinder were to have female threads of say 3/4 by 20 tpi, there is room
    for a short sleeve about 1/8 inch thick to hold the rod and pendulum
    weight together.  The sleeve would be threaded inside and out at the same
    20 tpi to fit the rod and weight, and a couple of notches at the lower end
    would allow the sleeve to be screwed up or down by a hollow shaft
    screwdriver.  The relative location of the brass weight and the rod would
    remain the same, but the amount of temperature compensation would change,
    I think.  For example, as the sleeve is screwed downward, the top end of
    the brass weight would remain in the same position relative to the rod
    initially, but would move upward more as the temperature increases.  The
    change in position of the sleeve (lowered) would have to be compensated by
    adding a weight in your tray, I believe, or by screwing the brass weight
    upward a bit on the sleeve and rod.  Is this what you did?  If the Invar
    rod length were actually invariable in a moderate temperature range, I
    think placing the sleeve in the mid point of the brass cylinder would be a
    good start experimentally, but where would theory suggest?"
    It's an interesting suggestion. The pendulum bob is a lead-filled brass 
    cyclinder with a sleeve up the middle for the pendulum rod, which latter is 
    of Invar 9.5 mm diameter (3/8 inch in American). The end of the pendulum rod 
    is threaded for an adjusting nut. Sitting on top of the nut is a brass 
    compensating sleeve that slides freely in the bob and over the rod. The top 
    of the sleeve carries the weight of the bob at the bob's mid-point, so that 
    the expansion of the bob has no effect on compensation. As the invar expands 
    a little downwards with increase in temperature, the short brass compensating 
    sleeve expands up a lot, relatively, lifting the bob. If the length of the 
    sleeve is correct, the centre of mass of the pendulum stays where it was. If 
    I have undertood Richard correctly his suggestion would not work with my bob. 
    No changes in the relative lengths of the pendulum rod and compensating 
    sleeve would take place.
    The Synchronome clock, a variation of which gave us Greenwich mean time for 
    some years, used a solid pendulum of type metal (an alloy of lead, tin and 
    antimony)sitting directly on the adjusting nut, with an Invar pendulum rod. 
    The upward expansion of the bob itself took care of compensation. There are 
    several other  systems of compensation, but none that relies on screw threads 
    carrying the weight of the bob for adjustment is satisfactory for precision 
    clocks because .02 mm axial movement results in about 1 second a day change 
    in timekeeping for a 2 second pendulum: even a fine thread does not give 
    enough sensitivity. A further problem with a load bearing thread is that 
    machining irregularities prevent smooth adjustment (and it is difficult to 
    get a fine finish with standard Invar, a very tough nickel-iron alloy).
    But there is much more to precision time keeping than pendulum compensation. 
    For example, variations in the driving force that keeps the pendulum going 
    will result in changes in the length of its arc, as will changes over time in 
    the oil that lubricates the escapement. Variations in atmospheric pressure 
    and air density also set limits to what is achievable. Richard J Matthys' 
    book "Acurate Clock Pendulums" will tell you more than most people will wish 
    to know about the subject.
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