Recently I came across a problem on Searle's Experiment where we had to find the maximum possible error in Young's modulus calculation due to an error in the calculation of extension of string. Personally, I've never seen or done this experiment. My book says that the maximum error in extension in string calculation would be equal to the least count of the vernier callipers. I don't understand how is this possible. I've worked with a Vernier Callipers but I've never heard of anything like this before. Can someone brief the experiment for me and provide me with a satisfactory explanation?