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That part of a modern sextant used to measure the minutes part of the sextant angle. It is a small, drum-shaped part that is rotated by the navigator to read the arc minutes by aligning a scale printed on the drum with an index mark fixed to the arc of the sextant.
The scale printed on the micrometer drum is called the micrometer scale. It can usually be read to a precision of 1'. On most modern metal sextants, the tenths of minutes are read from the vernier scale that is printed on the arc of the sextant. Thus, the degrees part of the angle is read from a scale on the arc of the sextant; the minutes part read from the scale on the micrometer drum, and the tenths of minutes read from the Vernier scale on the arc of the frame.
Note that the "micrometer drum" is sometimes referred to as just the "micrometer" or the "micrometer scale" in context of sextant discussions. Note, too, that at least one classic metal sextant has a micrometer drum but not a vernier scale. With these units the tenths must be estimated by inspection alone.