An electro-optic system where an electrical potential is changed and/or regulated to achieve a desired potential applied to an electro-optic medium. The electro-optic system comprises an electro-optic medium disposed in a chamber defined in part by two electrodes. A power source is connected to the two electrodes and thereby operable to apply an electrical potential to the electrochromic medium across the two electrodes. A voltmeter is operable to measure an electrical potential between two points within the chamber. Additionally, a controller is connected to the voltmeter and the power source and is operable to regulate the electrical potential applied by the power source based, at least in part, on the electrical potential measured between the two points within the chamber by the voltmeter.