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:  Small neon bulbs will typically oscillate with values of "R" between 500kΩ and 20MΩ. If "C" is not small, it may be necessary to add a resistor in series with the neon bulb, to limit current through it to prevent damage when the capacitor discharges. This will increase the discharge time and decrease the frequency slightly, but its effect will be negligible at low frequencies. The period of oscillation can be calculated from the breakdown and extinction voltage thresholds of the lamp used. During the charging period, the bulb has high resistance and can be considered an open circuit, so the rest of the oscillator constitutes an RC circuit with the capacitor voltage approaching "V"S exponentially, with time constant "RC". If "v"("t") is the output voltage across the capacitor  Solving for the time Although the first period is longer than the others because the voltage starts from zero, the voltage waveforms of subsequent periods are identical to the first between "V"e and "V"b. So the period "T" is the interval between the time when the voltage reaches "V"e, and the time when the voltage reaches "V"b  This formula is only valid for oscillation frequencies up to about 200 Hz; above this various time delays cause the actual frequency to be lower than this. Due to the time required to ionize and deionize the gas, neon lamps are slow switching devices, and the neon lamp oscillator is limited to a top frequency of about 20 kHz. The breakdown and extinction voltages of neon lamps may vary between similar parts; manufacturers usually specify only wide ranges for these parameters. So if a precise frequency is desired the circuit must be adjusted by trial and error. The thresholds also change with temperature, so the frequency of neon lamp oscillators is not particularly stable. Like other relaxation oscillators, the neon bulb oscillator has poor frequency stability, but it can be synchronized (entrained) to an external periodic voltage applied in series with the neon bulb. Even if the external frequency is different from the natural frequency of the oscillator, the peaks of the applied signal can exceed the breakdown threshold of the bulb, discharging the capacitor prematurely, so that the period of the oscillator becomes locked to the applied signal.