I love spinning! But one of the problems with spinning is gage. How do you know if you're spinning too thick or too thin? You don't want to have to spin up 100 grams or more, then ply, then measure and weigh... And what do you do when you want to spin yarn for a particular project ... how do you spin for that project which calls for a commercially spun yarn?
I use two different "tools" : WPI and the McMorran Balance. WPI stands for "wraps per inch" and simply enough you see how many times you can wrap your yarn around a ruler in one inch. For example, many socks require a yarn from 15 to 18 wpi. So that's pretty easy... particularly if you are looking at a pattern that mentions wpi.
But for many handspinners, we really want to know how many meters (or yards) we will have in 100 grams (or one pound) of yarn. H. McMorran, a lecteur in textile testing in Scotland came up with a clever scale that tells you just that!