strange remark about your collection?

That's actually not quite true. There is an absolute voltage reference in the form of electrical neutrality, with objects with an electron deficit being positive to it and objects with an electron excess being negative to it. However, for voltage magnitudes likely to be encountered outside an electrostatics setup, absolute neutrality is hard enough to identify, and deviations from it so small, that your statement is an excellent approximation, certainly good enough for practical purposes. :-) The resulting matrix _may be_ non-singular. There are two ways it can be singular: if there is a dead short between any two voltage nodes, in which case any attempt to solve the matrix will result in division by zero at some point, or if there is a part of the circuit with no connection to any voltage nodes, in which case all voltages in that portion are indeterminate. I ran into those conditions when building my resistor-blob solving program - that's why I'm aware of them. :) As I mentioned later, it evaluates to the _local_ speed of light (always <= c, with equality only in a vacuum). Talking with the guy, it turned out the shirt came from some undergrad physics club or some such, and they wanted to come up with something at least a little bit more interesting than just another way to write c. :-) Yes. I was being somewhat inaccurate in the interests of brevity (at the time, I mean, not when reporting the event now). I'm not sure. I am and was not physicist enough to really understand, nor explain, the formula. I discussed it briefly with him, but not in enough detail to cover things like that. /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B