Fluorescent lamps are usually operated with an AC discharge whose frequency is set by the power supply-60 Hz in the United States. However, it has been found that the tube has a higher efficacy if it is operated at a high frequency, for example 20-30 kHz. The reason for this increase in power is that there is less time between field reversals for the ions to collide with the electrodes, and so the rate of energy loss through electrode collision is reduced. Operation at high frequency requires a transistorized ballast, which has the added advantage that the lamp can be dimmed, unlike low frequency lamps where the current and voltage to the tube are fixed and the tube cannot be dimmed.
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