An insert to be inserted into a circular-shaped petri dish for generating a substantially uniform electric field across the petri dish filled with a fluid establishing a salt bridge, comprising: a circular-shaped bottom plate configured to be fitted into the circular-shaped petri dish, the bottom plate defining a circular-shaped space of a substantially uniform thickness together with a circular-shaped bottom surface of the petri dish as a channel accepting said fluid when the insert is inserted into the petri dish; a side channel vertically erecting from a circular periphery of said bottom plate, the side channel communicating with said circular shaped space defined by the bottom plate so as to accept said fluid; and a pair of current rectifying chambers each having a generally planar shape connecting to and communicating with the side channel, the current rectifying chambers being diagonally disposed to face each other along a diameter of the circular shaped bottom plate and being line-symmetric with each other with respect to said diameter, each of the current rectifying chambers extending inwardly from a respective portion of a top end of the side channel and having a salt bridge port to accept the fluid and provide the fluid to said circular-shaped space defined by the bottom plate through said side channel so as to establish the salt bridge, wherein portions of the side channel other than the portions thereof respectively connecting to the current rectifying chambers each have a generally concave top profile having a lowest point at a center between the pair of current rectifying chambers and are each shaped 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 direction of said diameter in a substantially entire area thereof. a circular-shaped bottom plate configured to be fitted into the circular-shaped petri dish, the bottom plate defining a circular-shaped space of a substantially uniform thickness together with a circular-shaped bottom surface of the petri dish as a channel accepting said fluid when the insert is inserted into the petri dish; a side channel vertically erecting from a circular periphery of said bottom plate, the side channel communicating with said circular shaped space defined by the bottom plate so as to accept said fluid; and a pair of current rectifying chambers each having a generally planar shape connecting to and communicating with the side channel, the current rectifying chambers being diagonally disposed to face each other along a diameter of the circular shaped bottom plate and being line-symmetric with each other with respect to said diameter, each of the current rectifying chambers extending inwardly from a respective portion of a top end of the side channel and having a salt bridge port to accept the fluid and provide the fluid to said circular-shaped space defined by the bottom plate through said side channel so as to establish the salt bridge, wherein portions of the side channel other than the portions thereof respectively connecting to the current rectifying chambers each have a generally concave top profile having a lowest point at a center between the pair of current rectifying chambers and are each shaped 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 direction of said diameter in a substantially entire area thereof.