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    Re: Pendulous gyroscopes
    From: Peter Fogg
    Date: 2002 Sep 17, 08:38 +1000

    Peter. The gyroscopes I referred to were suspended on a very fine invar
    wire, with the spin axis horizontal. With the instrument pointed
    approximately North, in low to medium latitudes,  the gyroscope would slowly
    oscillate about the North direction. The gyroscope's movement was displayed
    on a small screen. The electrical supply leads were extremely fine wires.
    The accuracy of the determination of North was of the order of 15 seconds of
    arc. Cheers. George
    Brian Whatcott wrote:
    > I did not completely recognize the setup that George referred to,
    > when reading Peter's description.
    > I wonder if the method is similar to the principle of Sperry's North
    > seeking gyro-compass?
    > Here, if a gyro spins in the local vertical direction, it will tilt off
    > vertical,
    > until its erecting method reduces the corrections need by orienting it
    > to the geographic north axis.   So this device needs to sense the
    > steady rotation of the Earth. Which can be defeated in polar latitudes,
    >   by motion contra the Earth's spin.
    > Brian
    > At 07:13 PM 9/15/02, Peter Fogg, you wrote:
    > >'Peter. I did my PhD on pendulous gyroscopes.
    > >Part of my field wok was made
    > >  at a station in latitude S80 degrees. I found the latitude of this
    > >station from my gyroscope observations to within two
    > >  minutes of arc. Of course nothing works at the pole. George.'
    > >
    > >This was in response to a query from Walter about whether polar
    > >explorers used pendulums (please excuse any inaccuracy here but I can't
    > >find this posting). Was interested enough to seek more information (over
    > >the phone) and this is my summary of what I found out:
    > >
    > >A pendulous gyroscope spins at about 20,000 rpm at the end of a fine
    > >thread.  As it is lowered it oscillates in a plane parallel to the
    > >direction of (true) north. It is accurate at low and medium latitudes,
    > >but at high latitudes the period of oscillation increases and it becomes
    > >progressively less accurate (which explains why it was a feat to find
    > >'the latitude of this station (S80 degrees) from my gyroscope
    > >observations to within two minutes of arc'. At the pole it doesn't work
    > >at all, 'just flops about'.
    > >
    > >It has practical uses in surveying, and my George has used it in
    > >surveying in underground mines where below ground workings can be
    > >correlated to above ground ones without any direct link between them.
    > >
    > >He also mentioned that while on the Antarctic mainland he developed
    > >tables for the observing of daylight stars through the telescope of a
    > >theodolite, which is feasible for the most bright stars. I asked whether
    > >in this case binoculars could be used for observing daylight stars, the
    > >problem is firstly 'you need to know where to look' and secondly, such
    > >observations are difficult because of the instability of the hand-held
    > >binoculars. This is why his instrument of choice for land based
    > >observations is the theodolite, rather than a hand-held sextant.
    > >Obviously these observations in Antartica were made in fine weather with
    > >full support, I don't mean to compare them with those of polar
    > >explorers.
    > >
    > >Any mistakes here are most likely to be mine.
    > Brian Whatcott
    >    Altus OK                      Eureka!

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