Which component of a micrometer rotates to allow measurement?

The thimble is the component of a micrometer that rotates to facilitate precise measurements. When measuring, the user turns the thimble, which is attached to a screw mechanism. This rotation moves the spindle closer to or further away from the anvil, allowing for very accurate measurements of small distances. The graduations on the thimble provide detailed measurement readings in combination with the sleeve, which remains stationary and offers a baseline for these measurements.

Other components, such as the sleeve, can provide fixed measurements but do not rotate to initiate measurement. The lock nut is used to secure the position of the micrometer after measurement has been taken, and the frame is the overall structure that holds the thimble and anvil in place. These parts serve important roles but do not have the function of rotating to allow for measurement adjustments. Thus, the thimble's unique ability to rotate makes it the correct answer.