A closed circuit is necessary for current flow. Andre Ampere, the French mathematician and physicist who quantified electric flow around about the year 1820. Resistance Opposition to the flow of current. Unit of measurement is called an Ohm, stated as R, or by the Greek letter omega. All conductors have some resistance, this is necessary because if a conductor had no resistance the connection would be a short circuit with excessive current flow.
Insulators have high resistance and conductors have low resistance.
Ohm's Law Named after the German physicist Georg Ohm who in 1827 described resistance to electrical flow. Expresses the fundamental relationship between voltage, current, and resistance. The amount of work being done. It is named after James Watt a Scottish engineer, 1736-1819. Electric power is simply the product of voltage times current.