Figure 3-7. Horizontal Circle and Micrometer (New Model)
(2) If the instrument is slightly out of adjustment, the index mark may point halfway between the
circle marks and cause confusion. In this case, obtain a reading by locating the degree graduation to the
left of the index and counting to the right from the 4-minute subdivisions until reaching the diametrically
opposite degree mark on the upper, inverted circle image. Multiply the number of these subdivisions by
one-half their value to obtain the proper number of minutes read by the index.
(3) One method of reading the minute value of the horizontal circle uses both the upper and
lower circles. If both circles are used, as shown in Figure 3-8, the number of divisions or graduations
from the degree mark on the lower circle to the degree mark diametrically opposite (180,) on the upper
circle, when multiplied by 2, gives the value of minutes.
Figure 3-8. Determining the Minute Value
When the 10-percent mark on the lower circle coincides with the 190, mark on the upper circle,
the minute value is 0. Counting to the right from the 9, mark on the lower circle to the 189, mark on the
upper circle, there are 30 divisions. The actual value is 1000' which is equal to 960'. Therefore, in 60
minutes, which is the minute value from 9, there are 3 divisions. The minute value of each graduation