until the 1960s sawtooth oscillators were also built with thyratrons. These were gas-filled triode electron tubes. These worked somewhat similarly to neon bulbs, the tube would not conduct until the cathode to anode voltage reached a breakdown voltage. The advantage of the thyratron was that the breakdown voltage could be controlled by the voltage on the grid. This allowed the frequency of the oscillation to be changed electronically. Thyratron oscillators were used as time bases in oscilloscopes. Operation In the Pearson-Anson oscillator circuit (top) a capacitor C is connected across the neon bulb N The capacitor is continuously charged by current through the resistor R until the bulb conducts, discharging it again, after which it charges up again. The detailed cycle is illustrated by the hysteresis loop abcd on the current-voltage diagram at right: When the supply voltage is turned on, the neon bulb is in its high resistance condition and acts like an open circuit. The current through the resistor begins to charge the capacitor and its voltage begins to rise toward the supply voltage. When the voltage across the capacitor reaches b, the breakdown voltage of the bulb Vb, the bulb turns on and its resistance drops to a low value. The charge on the capacitor discharges rapidly through the bulb in a momentary pulse of current (c). When the voltage drops to the extinction voltage Ve of the bulb (d), the bulb turns off and the current through it drops to a low level (a). The current through the resistor begins charging the capacitor up again, and the cycle repeats. The circuit thus functions as a low-frequency relaxation oscillator, the capacitor voltage oscillating between the breakdown and extinction voltages of the bulb in a sawtooth wave. The period is proportional to the time constant RC. The neon lamp produces a brief flash of light each time it conducts, so the circuit can also be used as a "flasher" circuit. The dual function of the lamp as both light source and switching device gives the circuit a lower parts count and cost than many alternative flasher circuits. Conditions for oscillation The supply voltage VS must be greater than the bulb breakdown voltage Vb or the bulb can never conduct. Most small neon lamps have breakdown voltages between 80 and 150 volts. If the supply voltage is close to the breakdown voltage, the capacitor voltage will be in the