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.