A disclosed insert for a circular-shaped petri dish can generate a substantially uniform electric field across the petri dish that is filled with a fluid establishing a salt bridge. The insert includes a circular-shaped bottom plate defining a circular-shaped space; a side channel vertically erecting from a circular periphery of said bottom plate; and a pair of current rectifying chambers each having a generally planar shape communicating with the side channel. In at least some aspects of the invention, portions of the side channel bridging the pair of current rectifying chambers each have a generally concave top profile having a lowest point at the center between the pair of current rectifying chambers such that, when the salt bridge is established, the circular-shaped space defined by the bottom plate exhibits a substantially uniform electric field in a substantially entire area of the space.