How can I calculate electrode impedance using my intracellular amplifier?

Most amplifiers have a knob or switch that supplies a current pulse to the electrode and allows you to measure the electrode resistance within LabChart software. For example, the AM3100 Intracellular Amplifier features the switch described in the manual excerpt below:

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When this switch is activated, the voltage signal from the Output of the amplifier will be proportional to the electrode resistance (10 mV/M-ohm). For example, if the voltage shown on the screen is 12 mV, the electrode resistance is 1.2 M-ohms.