What statement about black light bulbs of the mercury arc type is false?

The statement indicating that line voltage variations above 120 V will have little or no effect on the bulb is false. Mercury arc black light bulbs, similar to other types of discharge lamps, are sensitive to fluctuations in voltage. When the line voltage increases significantly, it can affect the performance and lifespan of the bulb. High voltage can lead to excessive current, which may cause the bulb to use more energy than necessary, potentially damaging it or causing inconsistent performance.

In contrast, the other statements reflect accurate characteristics of mercury arc black light bulbs. High voltage does enhance performance by allowing the bulb to operate more efficiently, providing a stronger output of ultraviolet light. The claim about them being ineffective under UV light relates to their operational context; they are designed to produce UV light rather than be effective in it. Finally, mercury arc bulbs indeed require a warm-up period to reach their maximum intensity due to the nature of gas discharge and achieving the necessary temperature for optimal output.