What is the primary use of a dial indicator?

A dial indicator is primarily used to measure distance or variation in distance with high precision. It consists of a calibrated dial that displays the measurement, making it an essential tool for tasks like checking the flatness of surfaces, measuring the straightness of a shaft, or detecting small amounts of wear or play in mechanical components. The indicator operates by translating linear motion into a rotation displayed on the dial, which provides accurate readings down to thousandths of an inch or hundredths of a millimeter.

In contrast, the other options refer to different types of measurements that are not the primary functions of a dial indicator. For example, measuring torque involves different tools such as torque wrenches, while pressure measurements typically require a manometer or pressure gauge. Measuring angles is done using tools like protractors or angle finders. Hence, the primary function of the dial indicator clearly aligns with measuring distance.