You can use a micrometer to measure very small distances with extreme accuracy. To measure the distance, you simply turn a screw until the area to be measured is fitted perfectly to the mouth of the micrometer. Most micrometers are outside micrometers, which have a frame with a mouth that widens very slowly as you turn the screw. Inside micrometers, on the other hand, lack this frame, and the entire micrometer fits inside of the area to be measured. Inside micrometers are especially helpful for measuring the insides of pipes and other circular objects.

Add extension rods onto the micrometer one by one. Try each one from smallest to largest until you reach the largest one that will still allow the micrometer to fit into the area to be measured.

Hold the reference end of the micrometer against one side of the area to be measured.

Turn the screw on the micrometer to lengthen it slightly. Continue doing this until the head end of the micrometer hits the other end of the area to be measured.

Adjust the micrometer slightly, making sure that it is measuring the exactly area that you want. (It should not be off-center or diagonal from any perspective.) You may need to adjust the screw further while you get the micrometer to the perfect angle.

Remove the micrometer from the area and look at the distance displayed on the sleeve. Add that distance to the length of any extension rods attached to the micrometer to get a precise measurement.