The resistance between two terminals of an electrical device or circuit is determined by its current–voltage (I–V) curve (characteristic curve), giving the current i through it for any given voltage v across it. Most materials, including the ordinary (positive) resistances encountered in electrical circuits, obey Ohm's law ; the current through them is proportional to the voltage over a wide range. So the I–V curve of an ohmic resistance is a straight line through the origin with positive slope. The resistance is the ratio of voltage to current, the inverse slope of the line (in I–V graphs where the voltage v is the independent variable) and is constant.