You can verify your Plate Gap Voltage by placing the probes of an electrical meter on two adjoining electrodes, as shown in the following picture:
Always place the positive lead on an electrode that is on the same side as a positive elecrode. Place your negative lead on an electrode that is on the same side as the negative electrode.
Example: If you were to measure from the middle neutral electrodes, then the negative would be on the left and the positive would be on the right as shown above. This will keep your polarity correct on your meter. You can measure the voltage from any two adjoining electrodes in this way to determine your plate gap voltage. You should have the same voltage for each plate gap.