An outside micrometer has a negative error. The correct reading can be taken by:

Explanation:

Micrometer:

A micrometer is a precision instrument used to measure a job, generally within an accuracy of 0.01 mm. 

• Micrometers used to take the outside measurements are known as outside micrometers.
• The outside micrometer has a negative error
• An inside micrometer is used to measure the diameter of holes.

If on bringing the anvil and spindle of micrometer screw gauge together, the zero mark of the circular scale is above the main scale line, then the zero error is said to be negative.

Outside micrometers are available in ranges of 0 to 25 mm, 25 to 50 mm, 50 to 75 mm, 75 to 100 mm, 100 to 125 mm and 125 to 150 mm.

Important Points

Reading micrometer measurements:

For all ranges of micrometers, the graduations marked on the barrel is only 0-25 mm. 

First, note the minimum range of the outside micrometer. While measuring with a 50 to 75 mm micrometer, note it as 50 mm. Then read the barrel graduations. Read the value of the visible lines on the left of the thimble edge.
13.00 mm (Main divison reading on barrel) + 00.50 mm (Sub division reading on barrel) = 13.50 mm (Main division + sub - division value)

Read the thimble graduations:

Read the thimble graduations in line with the barrel datum line, 13th div.

Multiply this value with 0.01 mm (least count). 13 x 0.01 mm = 0.13 mm.
On addition,
Minimum range 50.00 mm
Barrel reading 13.50 mm
Thimble reading 00.13 mm
Total = 50 + 13.50 + 0.13 = 63.63 mm
The micrometer reading is 63.63 mm.