Which instrument would NOT be suitable for measuring the depth of a hole?

Gauge blocks are precision measuring tools typically used to calibrate measuring equipment or as references for length and can provide a very accurate standard for measurement. However, they are not designed for measuring the depth of holes directly. Instead, depth micrometers, dial calipers, and steel rules are all suitable for this purpose.

A depth micrometer is specifically designed to measure the depth of holes and recesses, making it very effective for this task. Dial calipers can also measure depth (using the depth rod) and are versatile tools for a range of measurements. Steel rules, while not as specialized, can still be effectively used to measure the depth of shallow holes if used properly.

Therefore, while gauge blocks serve an important function in precision measurement, they do not lend themselves to measuring the depth of holes, making them the least suitable option in this context.