the resistor r must also be within a certain range of values for the circuit to oscillate. this is illustrated by the load line ("blue") on the "iv" graph. the slope of the load line is equal to r. the possible dc operating points of the circuit are at the intersection of the load line and the neon lamp's "iv" curve ("black") in order for the circuit to be unstable and oscillate, the load line must intersect the "iv" curve in its negative resistance region, between "b" and "d", where the voltage declines with increasing current. this is defined by the shaded region on the diagram. if the load line crosses the "iv" curve where it has positive resistance, outside the shaded region, this represents a stable operating point, so the circuit will not oscillate: