Re: The Wild T4 Theodolite - Don't Sneeze!
From: Paul Hirose
Date: 2016 Apr 02, 22:11 -0700
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 sec) 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 reading. > (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. http://www.sage.unsw.edu.au/currentstudents/ug/projects/f_pall/html/ US Army manual TM 5-442, Precise Astronomic Surveys, has much information on determination of latitude, longitude, and azimuth with the Wild T4. https://archive.org/details/armytechnicalman003486mbp 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. http://www.photolib.noaa.gov/brs/geind22.htm http://www.photolib.noaa.gov/htmls/geod1128.htm http://www.photolib.noaa.gov/htmls/geod1129.htm http://www.photolib.noaa.gov/htmls/theb1537.htm http://www.photolib.noaa.gov/htmls/theb1538.htm