We report here an improvement on the so-called "Hammer Method" of making capacitive measurements of cathode fall in fluorescent lamps. E, E. Hummer~(-4) has described a capacitively-coupled technique for measuring cathode fall in 60-Hz AC operated fliiorescenl lamps. In this method, a capacitive probe is wrapped around the end of the lamp, and picks up the fluctuations of plasma potential in front of the electrode and presents them to an oscilloscope for detection. The negative glow in front of the cathode is approximately an equipotential blob of high-density plasma at a potential positive relative to the cathode, equal to the cathode fall. Thus, the fluctuations of plasma potential on the cathode half cycle can be interpreted as the signature of the fluctuations of cathode fall during the half cycle.
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