One measuring instrument that is used a lot in the manufacturing industry is the micrometer.  These are also very popular with machinists making quality pieces.  In class we are using outside micrometers; this means that the instrument is intending to measure the outside dimensions of objects.

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Here is a great reference on how to read the measurement from the instrument.  

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There are other resources available as well.  As with so many things we do, practice makes all the difference in reading these instruments accurately.