What is the best instrument for measuring to the thousandth of an inch?

A micrometer is the best instrument for measuring to the thousandth of an inch due to its design and precision capabilities. It typically features a calibrated screw mechanism that allows for very fine adjustments, enabling measurements with high accuracy, often down to 0.001 inches or even finer. This level of precision makes micrometers particularly suitable for applications in machining, engineering, and other fields where precise measurements of small dimensions are critical.

In contrast, the other instruments listed serve different functions. A tachometer measures rotational speed and cannot provide linear measurements of physical dimensions. A caliper, while also capable of measuring dimensions, is generally less precise than a micrometer, as it can typically measure in increments of 0.01 inches or 0.001 inches, but with less assurance of accuracy compared to the micrometer. A pyrometer measures temperature, which is unrelated to dimensional measurements. Therefore, the micrometer stands out as the ideal choice for achieving the required precision of a thousandth of an inch.