The 3143 obtains ground resistance Rx by measuring the loop impedance of the closed path that goes from the lead wire to earth, then back to a return line which is strung along the surface of the earth. By varying the frequency of the measurement signal source so that LCR is serially resonant, the ground resistance Rx can be determined as follows: Rx = Vm / (Vc - Vm) × Ro. Here, Vc is the signal source voltage, Vm is the voltage between the measurement terminals, and Ro is the signal source output resistance.

With the 3143 you turn the measurement knob to vary the signal source frequency. As the knob is turned, the smallest value that appears (the value at the resonance point) is the ground resistance value Rx.

Common ground resistance meters such as the 3151 measure the resistance using a frequency between 500 Hz and several kHz. The 3143 uses a frequency between 100 kHz and 1.5 MHz, close to that of a lightning surge.