Ground is typically considered to be at 0 volts, serving as a reference point for measuring voltage in a circuit. A bulb will not glow if there is no potential difference between the terminals, as current cannot flow without it. When one source provides 5 volts and another provides 15 volts, the bulb will glow due to the 10-volt potential difference created. Ground can technically be set at any voltage, but it is most practical to use 0 volts for clarity. Ultimately, the key factor for current flow and bulb illumination is the existence of a voltage difference.