Which tool is primarily used for precise measurements of small dimensions?

The primary tool used for precise measurements of small dimensions is a micrometer. This instrument is specifically designed to measure the thickness or diameter of small objects with a high degree of accuracy, often within a thousandth of a millimeter. Micrometers utilize a screw mechanism that allows for fine adjustments, making it possible to measure very small dimensions reliably.

In contrast, calipers can measure internal and external dimensions but typically do not provide the same level of precision as micrometers. While they can be quite accurate and are useful for a variety of measurements, they are generally less suited for measuring extremely small dimensions compared to a micrometer. A ruler, although useful for longer distances and general measurements, does not offer the fine level of detail necessary for precise small measurements. A protractor is primarily used for measuring angles rather than linear dimensions, making it irrelevant for the context of measuring small dimensions. Thus, the micrometer stands out as the ideal choice for tasks requiring meticulous precision.