Hello all, I have a petri dish 8.7 cm in diameter filled with 20 mL of TSB media. I have set up a .7mm graphite at the 0cm mark of the diamter and measured current at 4.6Volts over the entire distance of the petri dish (8.7cm) with .5 cm step sizes. For example: When the probes are 1cm apart I am reading a current of 2.8mA. I was wondering how I can calculate a current density for all the known information. Current Density (J) = Current (I) / Area (A) How could I calculate the area if I am 1 cm apart in a 8.7 cm petri dish?