The streaming current detector (SCD) is an instrument used for continuous characterization of colloidal surface charge. It measures the alternating current generated by mobile countering when charged material momentarily adheres to cylinder and piston walls while the piston reciprocates, The utility of the SCD for coagulant control is generally accepted, as evidenced by the estimated one thousand units in service in North America, However, one drawback to this device has been the lack of an exact quantitative model explaining its functioning, This paper provides such a description, relating the generated current to the zeta potential of the characterized material as a function of instrument parameters and solution chemistry, The model, programed in Mathematica, agrees closely with an earlier analytical solution under many conditions, Its inclusion of an inertial term, however, allows behavior of the SCD to be simulated under a more general range of conditions.