Two electrodes in water are connected to a constant-voltage source. A probe is connected to a multimeter. As the probe is moved around in the water, the multimeter shows the electric potential of the water at the probe tip relative to one of the electrodes.
To use the movie, open it in video analysis software. Choose a potential, such as 3 Volts. As you step through the video frame-by-frame, mark a point on the probe tip each time the multimeter reads 3 V. These marked points lie along the 3 V equipotential line.