HORIZONTAL CIRCLE

Figure 11-21.-Circle-reading optical system.

Engineering Aid 3 - Beginning Structural engineering guide book
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circle (fig. 11-22) with lines spaced at 20-min intervals, every third line numbered to indicate a degree, and the image of the micrometer scale on which the unit minutes and seconds are read. The numbers increase in value (0⁰to 360⁰, clockwise around the circle. The coincidence knob on the side of, and near the top of, the right standard is used in reading either of the circles. The collimation level and its tangent screw are used when the vertical circle is read. The circles of the theodolite are read by the COINCIDENCE METHOD in which optical coincidence is obtained between diametrically opposite graduations of the circle by turning the MICROMETER or COINCIDENCE KNOB. When this knob is turned, the images of the opposite sides of the circle appear to move in opposite directions across the field of the CIRCLE-READING MICROSCOPE. The grad- uations can be brought into optical coincidence and appear to form continuous lines crossing the dividing line. An index mark indicates the circle graduations that are to be used in making the coincidence. The index mark will be either in line with a circle graduation or midway between two graduations. The final coincidence adjustment should be made between the graduations in line with the index mark or when this index mark is halfway between the two closest graduations. HORIZONTAL CIRCLE.— To read the HORIZONTAL CIRCLE, turn the INVERTER or CIRCLE-SELECTOR KNOB until its black line is horizontal. Adjust the illuminating mirror to give uniform lighting to both sections of the horizontal circle; the micrometer scale is viewed through the circle-reading microscope. Focus the microscope eyepiece so that the graduations are sharply defined. The view through the microscope should then be similar to figure 11-22, view A. From this point, continue in the following way: 1. Turn the coincidence knob until the images of the opposite sides of the circle are moved into coincidence. Turning this knob also moves the mi- crometer scale. The view through the microscope now appears as shown in figure 11-22, view B. 2. Read the degrees and tens of minutes from the image of the circle. The nearest upright number to the left of the index mark is the number of degrees (105). The diametrically opposite number (the number ± 180⁰) is 285. The number of divisions of the circle between the upright 105 and inverted 285 gives the number of tens of minutes. In figure 11-22, view B, there are five divisions between 105 and 285; and the reading, therefore, is 105⁰50´. The index may also be used for direct reading of the tens of minutes. Each graduation is treated as 20 min. Thus, the number of graduations from the degree value to the index mark multiplied by 20 min is the value. If the index falls between graduations, another 10 min is added when the tens of minutes is read directly. 3. Read the unit minutes and seconds below from the image of the micrometer scale. This scale has two rows of numbers below the graduations; the bottom row is the unit minutes and the top row, seconds. In figure 11-22, view B, the unit minutes and seconds are read as 7'23.5'' 4. Add the values determined in Steps 2 and 3 above. This gives 105⁰57'23.5''as the final reading. Figure 11-22.-View of a 1-second theodolite circle. 11-26

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