# Are there any 'clever' mathematical techniques for testing a micrometer?

I've just inherited a vintage British 25 mm micrometer, and it's precious to me.

You can test a spirit level by reversing it, and similarly you can for an engineer's square. We can test a right angle via the 3-4-5 formula. Absolute verticality can be proved with a rock on a string. And NASA can detect the constituents of exoplanet atmospheres thousands of miles away just by peeking at them. Is there some mathematical 'trick' that we can perform to test the accuracy of a micrometer?

I'm quite prepared to accept no.

This question excludes expensive gauge blocks.

• Measure something and use a second micrometer to get the dimension. Do take care with hand heating effects etc Commented Jan 3, 2021 at 18:26
• If it uses a screw thread, it is pretty much impossible for it to go "globally" out of calibration (how could you stretch the entire length of the screw?) but there may be local damage, or backlash in the screw. Get a set of feeler gauges where you can measure different combinations, and check if the thicknesses of two gauges measured separately are consistent with both measured together, etc. Commented Jan 3, 2021 at 20:11
• The good news/; there is nothing around an average home that needs to be measured that accurately. I am in the same situation with 0 to 1 and 1 to 2 in. mics of my fathers. I just realized they contributed to the WW 2 war effort, measuring drill bits . And more than a few of the bits went to the UK. I believe the "touch" of the operator in the use is more important than the absolute accuracy. Commented Jan 4, 2021 at 22:14