potential difference across it. 2) Use Ohms' law to find the equivalent resistance of the different combinations of resistors. 3) To distinguish between the Ohmic resistor and the nonOhmic resistor. Apparatus: DC Power supply, voltmeter, ammeter, resistors, and connecting wires. Theory and Background: The electric field inside a conductor equals zero when it is at electrostatic equilibrium, but when the charges move in a conductor they produce an electric current, which is defined by the current density (J). For an Ohmic material the ratio of the current density (J = I/A) and the electric field (E) is constant and equals the conductivity ( ). Hence Ohms' law can be written as: J = E