Due to soil resistivity and ground currents there exists a voltage between points a-d called common mode voltage denoted as Ecm The path of ground loop current is -a. Now Rt represents transducer impedance and in as input impedance measuring instrument. Due to common mode voltage, the current also flows through path a e-f-c-d-a. But in practice Zin >> Rt, hence the current through path an-e-f-c-d-a is very small compared to current through path a-b-cd-a. Still due to the two different currents, a voltage will exist across points be. This voltage is considered to be in series with transducer voltage. This is called series mode voltage. This the common mode voltage Ecm is responsible to generate series mode voltage. This series mode voltage can cause serious errors in the measurement.