Editing Micrometer (device) (section)

===Metric system===
[[File:578metric-micrometer.jpg|thumb|Micrometer thimble with a reading of <!--DO NOT CHANGE TO 5.78. IF YOU ZOOM IN YOU CAN CLEARLY SEE THE ESTIMATED TENTH IS BETTER AT 9 THAN 0. ALSO THE READING WOULD BE 5.780, NOT 5.78!-->5.779&nbsp;±&nbsp;0.005&nbsp;mm. (You must enlarge the image to be able to read the scale to its fullest precision.) The reading consists of exactly 5.5&nbsp;mm from the main scale plus an estimated 0.279&nbsp;mm from the secondary scale.<!--Note the '9' is an estimated tenth. Although it would not be unreasonable to claim a reading of 0.280 mm for the secondary scale, close inspection shows the main scale line does not quite reach 0.280 mm, making 0.279 mm the best but even 0.278 mm is not unreasonable. These two alternative readings would be expressed as 5.780 mm and 5.778 mm. Expressing the result as 5.78 mm is incorrect unless there's reason not to use the instrument's full precision to measure the object.--> Assuming no zero error, this is also the measurement.]]

The spindle of an ordinary metric micrometer has 2 threads per millimetre, and thus one complete revolution moves the spindle through a distance of 0.5 millimetres. The longitudinal line on the sleeve is graduated with 1 millimetre divisions and 0.5 millimetre subdivisions. The thimble has 50 graduations, each being 0.01 millimetre (one-hundredth of a millimetre). Thus, the reading is given by the number of millimetre divisions visible on the scale of the sleeve plus the division on the thimble which coincides with the axial line on the sleeve.

As shown in the image, suppose that the thimble were screwed out so that graduation 5, and one additional 0.5 subdivision were visible on the sleeve. The reading from the axial line on the sleeve almost reaches graduation 28 on the thimble. The best estimate is 27.9 graduations. The reading then would be 5.00 (exact) + 0.5 (exact) + 0.279 (estimate) = 5.779&nbsp;mm (estimate). As the last digit is an "estimated tenth", both 5.780&nbsp;mm and 5.778&nbsp;mm are also reasonably acceptable readings but the former cannot be written as 5.78&nbsp;mm or, by the rules for [[significant figure]]s, it is then taken to express ten times less precision than the instrument actually has! But note that the nature of the object being measured often requires one should round the result to fewer significant figures than which the instrument is capable.