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Using a micrometer is ideal for measuring external diameters with high precision due to its design and functionality. Micrometers are specifically engineered to provide accurate measurements of small objects or features, often down to one-thousandth of a millimeter or even finer. This level of precision makes micrometers essential in fields such as machining and manufacturing, where small tolerances are crucial.

The tool consists of a calibrated screw that can be turned to bring two measuring faces into contact with the object being measured. The scale on the micrometer allows the user to read the measurement accurately, often with additional capabilities for measuring depth or thickness when equipped with the appropriate surfaces.

While calipers can also measure external diameters, they are not as precise as micrometers. Calipers typically provide a less fine-tuned measurement, making them more suitable for general measurements rather than tasks requiring the highest accuracy. A thread gauge is used specifically for measuring screw threads, and a Vernier scale is a measuring device that can provide enhanced accuracy compared to a standard ruler, but is still not as precise as a micrometer for measuring small, external dimensions. Thus, for high-precision external diameter measurements, the micrometer stands out as the most effective tool.