Electric Currents Boundary Conditions Question

Hello,

I am a relatively new user of Comsol, and I am trying to learn to use the electric currents Physics module. I want to model two copper cylinders with a contact resistance, and 1V applied across the whole structure. I also want to see what the electric potential around this structure would look like in air.

If I do not have a box of air around the cylinders, Comsol solves the potential drop along the cylinders with no problem, but when the air box is introduced, a "Singular Matrix" error shows up**, which I understand is usually due to poorly formed boundary conditions.

How can I form boundary conditions to have Comsol complete this simulation? Also, in general, what do I need to be careful with in the electric currents module?

Thank you so much,
Robert



**The error is:
Failed to find a solution.
Singular matrix.

There are 21056 void equations (empty rows in matrix) for the variable mod1.V
at coordinates: (-3.5355,-3.53557,-5), (-3.17193,-3.86508,-5), (-2.77782,-4.15737,-5), (-4.15732,-2.7779,-5), (-3.86502,-3.17201,-5), ...
and similarly for the degrees of freedom (empty columns in matrix)
Returned solution is not converged.