Which instrument is described as a linear measuring instrument consisting of a scaled rule with a projecting arm at one end to which is attached a sliding vernier with a projecting arm that forms an arm with the other projecting arm?

• A. Vernier caliper
• B. Micrometer
• C. Dial caliper
• D. Magnetic base

Answer: (A)

This is a vernier caliper, a linear measuring instrument that blends a fixed main scale with a sliding vernier scale to interpolate measurements between main-scale divisions. The fixed jaw extends from one end of the scale, while a sliding jaw carries the vernier. As you slide, the zero of the vernier aligns with a line on the main scale, and you read the whole-number part from the main scale plus the fractional part from the vernier to get a precise measurement. This setup allows measuring external dimensions, internal dimensions, and depth with high precision.

A micrometer also measures linearly but uses a calibrated screw and a thimble, not a sliding vernier. A dial caliper reads the measurement on a dial rather than a vernier scale. A magnetic base is simply a stand or mounting device, not a measuring instrument.