Matthias Schabel schrieb: [Snip]
But in most ways differences behave more like what we expect when we use units. That is a length, for example, is really a difference between the two endpoints of an object, etc...
Exactly, a voltage is a difference between two potentials and a potential is a voltage measured against ground... The point I'm trying to make is that quantities of the same unit form a vector space and a vector space always has an basis. Selecting a particular basis is typically application dependent. Kelvin and Celsius seem to be two of the rare exceptions where a unit implies an origin. The wrapper class seems to be a good approach, but the problem of distinguishing several bases applies to all vector spaces, so we should be able to find a general solution. Andreas