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    Re: The Wild T4 Theodolite - Don't Sneeze!
    From: Paul Hirose
    Date: 2016 Apr 02, 22:11 -0700

    On 2016-03-29 20:13, N Drummond wrote:
    > The Wild T-4 instrument can read directly to .1 second (that's 1 in  12.96 
    million divisions).  Key to this capabilty was a very large glass horizontal 
    circle, an impersonal eyepiece micrometer to read the horizontal glass 
    circle, and of course the incredibly precise optical and mechanical 
    fabrication capability of the Wild company.
    The impersonal micrometer measures the transit of a star through the
    field of view during longitude determinations. It has nothing to do with
    reading the circles.
    In the old days a star moved across a set of stationary crosswires. Each
    time it coincided with a wire the observer pressed a switch. Due to
    variations in reaction time and visual perception, each observer had a
    systematic bias, or "personal equation".
    The impersonal micrometer replaces the fixed wires with a movable wire.
    Rotation of the micrometer screw to keep the wire on the star also
    rotates a disk with electrical contacts in its edge. A chronograph
    records the contact closures and the 1-second pulses from the clock as
    pen traces on a moving piece of paper.
    > - the large horizontal circle (240mm, or 24cm, or 9.5 inches)  provided a 
    basic instrument capable of 2-second readings  by scaling the same design 
    capability already in use with the Wild T-2 and T-3.    For comparison, the 
    Wild T-2 can be read directly to .4 seconds, with a horizontal circle of 
    8.5cm; the T-4 design uses a glass circle 3X larger with the abilty for 
    measuring about 3.3X greater divisions compared to the T-2 (.1 sec   vs .4 
    The T2 circle has 10 minute graduations. Its micrometer is directly
    readable to 1 second, i.e., that's the interval of the graduations.
    (There was also a T2 version which measured angles in gons.)
    A direct reading to .4 second isn't going to happen since that's not a
    neat subdivision of a degree.
    > - the optical micrometer at the eyepice further divided the 2-second reading 
    into 20 divisions-  so 2-seconds, divided into 1/20- became .1 second direct 
    > (think of using a microscope with 20-divisions/ intervals across the 
    field-of-view  to read the horizontal circle markings)
    > The key technology that provided this capability is due to the use of 
    photo-lithographic methods to mark the horizontal circle, not mechanical 
    engraving- The T-4, with a 240mm (24cm/ 10inch) wide circle, had a lot more 
    room for photo-lithographically etching the needed markings to allow 2-second 
    graduations on the circle.
    No, that's completely wrong. The T4 horizontal circle is graduated every
    2 *minutes* (a logical extension of the T3 circle, which is about half
    the diameter and graduated every 4 minutes). In the reading microscope,
    opposite sides of the circle are visible simultaneously, one above the
    other in a split image. The lower half is inverted and moves right when
    the upper half moves left. Thus the graduations in the split image
    coincide every minute and the circle is directly readable to 1 minute,
    sans micrometer.
    The micrometer has a 1 minute range. An optical mechanism shifts the
    halves of the split image in opposite directions when the micrometer
    knob is rotated. When the graduations coincide, degrees and minutes are
    read from the split image. Seconds are read from a dial (graduated every
    tenth), a portion of which is visible in the lower part of the
    microscope field.
    A T2 or T3 user will be immediately comfortable with this system. In
    fact, the micrometers on all three theodolites divide their range into
    600 parts.
    The University of New South Wales instrument collection includes a T4.
    In the photo, the impersonal micrometer drive knobs (one at each end of
    a common shaft) are visible at the left end of the horizontal axis. They
    could be mistaken for focus knobs. Below them, the horizontal circle
    reading microscope protrudes from the upright. It can be folded down
    when not needed. On the upright, at the base of the microscope, is the
    silver knob of the micrometer.
    The T2 and T2 have a knob to select either the horizontal or vertical
    circle. Both circles share the same microscope and micrometer. But these
    are separate on the T4. The vertical circle eyepiece is the little black
    thing at the extreme right of the picture. The micrometer knob is out of
    view on the other side.
    US Army manual TM 5-442, Precise Astronomic Surveys, has much
    information on determination of latitude, longitude, and azimuth with
    the Wild T4.
    The NOAA archive has some photos of the T4 in action. In the picture of
    the guy recording observations, I think the device with a paper strip
    coming out of a slot is a chronograph. The old fashioned chronographs
    made a helical trace on a rotating drum.

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