How to calculate a value between two points?

Hello,

I have modelled a simple 2D electric potential plot of a dielectric between two plates with a void in the middle. I want to know the voltage between across the void. I can get the plots on individual points on teh void but am unclear how the points are specified?

Does anyone know how to specify the function: voltage_diff = voltage at point 1 - voltage at point 2?

Thanks, David.