# Micrometer

## Instructions on use

- The micrometer is an extremely precise measuring instrument; the
reading error is 1/200 mm = 0.005 mm.
- Use the rachet knob (at the far right in the picture above) to
close the jaws
**lightly** on the object to be measured. It is not a
C-clamp! When the rachet clicks, the jaws are closed sufficiently.
- The tick marks along the fixed barrel of the micrometer represent halves of
millimeters.
- Every revolution of the knob will expose another tick mark on the barrel,
and the jaws will open another half millimeter.
- Notice that there are 50 tick marks wrapped around the moving barrel of
the micrometer. Each of these tick marks represents 1/100 millimeter.

- To read the distance between the jaws of the micrometer, simply add the
number of half-millimeters to the number of hundredths of millimeters.
In the example above, the jaws are opened (2.620 ± 0.005) mm,
that is, 5 half-millimeters and 12 hundredths of a millimeter.
- If two adjacent tick marks on the moving barrel look equally aligned
with the reading line on the fixed barrel, then the reading is half way
between the two marks. In the example above, if the 12th and 13th tick
marks on the moving barrel looked to be equally aligned, then the reading
would be (2.625 ± 0.005) mm.
- The micrometer may not be calibrated to read exactly zero when the jaws
are completely closed. Compensate for this by closing the jaws with the
rachet knob until it clicks. Then read the micrometer and subtract this
offset from all measurements taken. (The offset can be positive
**or**
negative.)
- On those rare occasions when the reading just happens to be a "nice" number
like 2 mm, don't forget to include the zero decimal places showing the
precision of the measurement and the reading error. So not 2 mm, but
rather (2.000 ± 0.005) mm.