Are there two sets of screws so that the coarse adjustment knob and the vernier adjustment knob can function separately?
Try Wikipedia to see how a micrometer works, and how a vernier works. There's a good little animation to make it perfectly clear. http://en.wikipedia.org/wiki/File:Micrometer_no_zero_error.gif
Do you have a picture of the specific micrometer you are referring to? Standard micrometers are manufactured with a single threaded spindle. The spindle will have 40 threads per inch which is why one graduation on the barrel is equal to .025" (1000 micro inch range/ 40 threads per inch=.025" per rotation). The barrel sleeve will also then have the vernier marked on it. The vernier uses the same spindle screw as the screws on good micrometers (like the ones we make ;-)) have a lead error of .000050"/inch or less so they are perfectly acceptable for a .0001" vernier scale. The vernier scale is set up so all of the spaces are N-1 graduations apart. What that means is that there are 10 spaces on the vernier in the same space that there are only 9 graduations on the barrel such that the gaps are all smaller on the vernier which is what allows you to take it out to a further decimal place. I know, clear as mud, right?