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    Leap seconds at Big Ben.
    From: George Huxtable
    Date: 2008 Dec 31, 19:56 -0000

    A bit more information about leap seconds, for anyone who happens to be
    interested.
    
    The chimes of Big Ben, from London, ring in the New Year, via radio and TV,
    all over Britain, and over much of the Western World that has so far escaped
    the dominion of the USA. The clock dates from nearly 150 years ago, the work
    of Nicholas Denison.
    
    Those responsible for the mechanism have the task of introducing a leap
    second, for which the mechanism was never designed. It wouldn't do to have
    the bells ring out a second before the due moment of New Year. My wife Joan,
    who takes an interest in such matters as her birthday is on New Year's Day,
    heard a news item on the radio about how the job is done, and has passed the
    information on.
    
    There is no seconds-hand on the clock, but presumably there must be an
    internal cam-wheel, turning as a seconds-hand would, to trigger the strike.
    The pendulum, nearly 4 metres long beats at 2-second intervals, a cycle
    taking 4 seconds, so that wheel would step at 2-second intervals.
    
    It wasn't made clear exactly how long before the moment of New Year the
    adjustment process for the leap-second commences, but the job has to be done
    gradually, as the required step-change is impossible. To adjust the rate,
    weights are added or subtracted from a tray that's attached to the pendulum.
    Traditionally, these have been (old) pennies, copper coins valued at a bit
    less han half of one (new) pence. This allows the clock to run a bit more
    slowly as the leap-second moment is approached, so that at New Year, it
    strikes correctly, allowing for the leap-second, and the rate of going can
    then be returned to normal. That must mean that during the adjustment
    period, the clock must be increasingly slow on UT, by up to ! second.
    Presumably, that will affect the strikings (at the quarter-hours) that
    precede midnight.
    
    A happy New Year to all,
    
    from George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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