For accurately measuring the shaft diameter of a motor to one thousandth of an inch, which instrument would be best?

The micrometer is the best instrument for accurately measuring the shaft diameter of a motor to one thousandth of an inch. This tool is specifically designed for precise measurements of small distances and has a thimble and a spindle that allow for fine adjustments, providing readings that can measure down to one thousandth of an inch (0.001 inches).

Micrometers are commonly used in mechanical applications where high precision is required, such as in manufacturing or mechanical maintenance. They can accurately measure cylindrical objects, like motor shafts, and often come with a scale that allows easy reading of the measurements.

In contrast, calipers can measure thicknesses and diameters but are generally less precise than micrometers and may not achieve the same level of accuracy. Wire gauges and rulers are not suitable for this level of precision, as wire gauges measure wire diameters in standard sizes and rulers provide only general length measurements with limited precision. Therefore, for the specific requirement of measuring to the thousandths of an inch, the micrometer is clearly the most appropriate choice.