Editing Micrometer (device) (section)

==Reading==
Micrometers are high precision instruments. Proper use of them requires not only understanding their operation itself but also the nature of the object and the dynamic between the instrument and the object as it is being measured. For simplicity's sake, in the figures and text below issues related to deformation or definition of the length being measured are assumed to be negligible unless otherwise stated.

===Customary/Imperial system===
[[File:276inch-micrometer.jpg|thumb|Imperial unit micrometer thimble showing a reading of 0.2760&nbsp;in. The main scale reads 0.275&nbsp;in (exact) plus 0.0010&nbsp;in (estimated) on the secondary scale (the last zero is an estimated tenth). The reading would be 0.2760&nbsp;±&nbsp;0.0005&nbsp;in, which includes plus/minus half the width of the smallest ruling as the error. Here it has been assumed that there is no zero point error (often untrue in practice).]]
The spindle of a micrometer graduated for the [[Imperial and US customary measurement systems]] has 40 threads per inch, so that one turn moves the spindle axially 0.025&nbsp;inch (1 ÷ 40 = 0.025), equal to the distance between adjacent graduations on the sleeve. The 25 graduations on the thimble allow the 0.025&nbsp;inch to be further divided, so that turning the thimble through one division moves the spindle axially 0.001&nbsp;inch (0.025 ÷ 25 = 0.001). Thus, the reading is given by the number of whole divisions that are visible on the scale of the sleeve, multiplied by 25 (the number of [[thousandth of an inch|thousandths of an inch]] that each division represents), plus the number of that division on the thimble which coincides with the axial zero line on the sleeve. The result will be the diameter expressed in thousandths of an inch. As the numbers 1, 2, 3, etc., appear below every fourth sub-division on the sleeve, indicating hundreds of thousandths, the reading can easily be taken.

Suppose the thimble were screwed out so that graduation 2, and three additional sub-divisions, were visible on the sleeve (as shown in the image), and that graduation 1 on the thimble coincided with the axial line on the sleeve. The reading would then be 0.2000 + 0.075 + 0.001, or 0.276&nbsp;inch.

===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.

===Vernier micrometers===
[[File:5783metric-micrometer.jpg|thumb|Vernier micrometer reading 5.783&nbsp;±&nbsp;0.001&nbsp;mm, comprising 5.5&nbsp;mm on main screw lead scale, 0.28&nbsp;mm on screw rotation scale, and 0.003&nbsp;mm added from vernier]]
Some micrometers are provided with a [[vernier scale]] on the sleeve in addition to the regular graduations. These permit measurements within 0.001&nbsp;millimetre to be made on metric micrometers, or 0.0001&nbsp;inches on inch-system micrometers.

The additional digit of these micrometers is obtained by finding the line on the sleeve vernier scale which exactly coincides with one on the thimble. The number of this coinciding vernier line represents the additional digit.

Thus, the reading for metric micrometers of this type is the number of whole millimeters (if any)  and the number of hundredths of a millimeter, as with an ordinary micrometer, and the number of thousandths of a millimeter given by the coinciding vernier line on the sleeve vernier scale.

For example, a measurement of 5.783&nbsp;millimetres would be obtained by reading 5.5&nbsp;millimetres on the sleeve, and then adding 0.28&nbsp;millimetre as determined by the thimble. The vernier would then be used to read the 0.003 (as shown in the image).

Inch micrometers are read in a similar fashion.

Note: 0.01 millimeter = 0.000393&nbsp;inch, and 0.002&nbsp;millimeter = 0.000078&nbsp;inch (78 millionths) or alternatively, 0.0001&nbsp;inch = 0.00254&nbsp;millimeters. Therefore, metric micrometers provide smaller measuring increments than comparable inch unit micrometers—the smallest graduation of an ordinary inch reading micrometer is 0.001&nbsp;inch; the vernier type has graduations down to 0.0001&nbsp;inch (0.00254&nbsp;mm). When using either a metric or inch micrometer, without a vernier, smaller readings than those graduated may of course be obtained by visual interpolation between graduations.