In a true ohmic device, the same value of resistance will be calculated from "R" = "V"/"I" regardless of the value of the applied voltage "V". That is, the ratio of "V"/"I" is constant, and when current is plotted as a function of voltage the curve is "linear" (a straight line). If voltage is forced to some value "V", then that voltage "V" divided by measured current "I" will equal "R". Or if the current is forced to some value "I", then the measured voltage "V" divided by that current "I" is also "R". Since the plot of "I" versus "V" is a straight line, then it is also true that for any set of two different voltages "V"1 and "V"2 applied across a given device of resistance "R", producing currents "I"1 = "V"1/"R" and "I"2 = "V"2/"R", that the ratio ("V"1 − "V"2)/("I"1 − "I"2) is also a constant equal to "R". The operator "delta" (Δ) is used to represent a difference in a quantity, so we can write Δ"V" = "V"1 − "V"2 and Δ"I" = "I"1 − "I"2. Summarizing, for any truly ohmic device having resistance "R", "V"/"I" = Δ"V"/Δ"I" = "R" for any applied voltage or current or for the difference between any set of applied voltages or currents.