Generally, a single arc spot appears on one side of each electrode, while a fluorescent lamp is operating. But there exist a certain condition under which the arc spot moves to the other side of electrode.
This condition was investigated by use of a discharge tube for experments as the first step of study. An arc spot appears on one side of each electrode where the potential difference between them is the largest. Therefore a difference of frequency between dischrge current and electrode voltage causes spot alternation.
There are two kinds of spot alternation. One of them is originated from the beat frequency between those of predicting two waves, the other is the variation of discharge current switched by electrode voltage. This paper deales with the frequency and amplitude about these two kinds of spot alternation.
Lastly high output ballasts are briefly described as one of applications of spot alternation.

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