If two, same-value, aluminum electrolytic capacitors
are connected in series, back-to-back with the positive
terminals or the negative terminals connected, the
resulting single capacitor is a non-polar capacitor with
half the capacitance. The two capacitors rectify the
applied voltage and act as if they had been bypassed
by diodes. When voltage is applied, the correct-polarity
capacitor gets the full voltage. In non-polar aluminum
electrolytic capacitors and motor-start aluminum
electrolytic capacitors a second anode foil substitutes
for the cathode foil to achieve a non-polar
capacitor in a single case.