So consider a resistor made up of one single, ohmic material. Thus: E(x,y,z) = ro(x,y,z) * J(x,y,z) because the material is ohmic and ro(x,y,z) = ro = ct because we have one single material. Thus: E(x,y,z) = ro * J(x,y,z) If E is uniform, then J is also uniform. But what is the condition for E to be uniform? Everywhere I see: ''Assuming E to be uniform etc etc.'' when deriving R = ro*L/S Must the resistor have a uniform cross-sectional area?