What tool would be used to measure the thickness of a solid metal rod?

An outside micrometer is the correct tool for measuring the thickness of a solid metal rod because it is specifically designed for measuring external dimensions with high precision. This tool consists of a calibrated screw that allows for accurate measurement of small lengths, typically down to one-thousandth of an inch (0.001"). It features a circular frame that holds a movable anvil and a fixed anvil, which clamp around the object being measured.

The outside micrometer's precision is crucial in metalworking and engineering applications where the exact dimensions of a rod need to be known for proper fitting or machining processes. The instrument's design allows users to take measurements by squeezing the micrometer around the rod and reading the value directly from the scale, ensuring accurate results.

In contrast, a depth gauge is primarily used for measuring depth, such as how deep a hole is, rather than thickness. A dial indicator is utilized for measuring small distances or displacements with a dial readout, effective for vertical movement and not suited for directly measuring the thickness of solid objects. An inside micrometer would measure internal dimensions, such as the diameter of a hole, making it unsuitable for measuring the thickness of a rod.