Electronic liquid level sensor

A pair of wires are coated in a conductive plastic sheath, and then imbedded in the inside of a plastic tube. The tube can be dropped down an oil well casing below the fluid level. A DC power source is applied across the wires, and current flows across the inside of the tube between the wires. The resistance drops between the wires in proportion to the fluid level inside the tube. Ohm's law allows a calculation of fluid level along the tube.