The requirement of a potential difference needed for a current to flow, and the need for an electric "pump" to continue the flow was related to how water flows in a fountain. The electric current is the flow of conducting electrons through a wire, with the net charge being zero. Amperes (AMPS) was listed as the units of measure for current, Voltage the unit of measure for potential difference and Ohms the unit for resistance. Voltage sources shown to be batteries or generators, which allows the charge to move steadily. Voltage causes current to flow.