Clear T17 mercury lamp - hollow cathode effect

A closeup shot of the lamp's electrode during the run-up phase reveals an interesting effect: the confinement of the (plasma) negative glow within the beehive structure of the electrode, whose geometry results in an enhanced ionization of the gas. This process, called the hollow cathode effect, increases the electron emission capacity of cathodes and thus raises their efficiency. This is a very useful effect that was used primarily in high-voltage sign tubes as well as SOX and many cold-cathode fluorescent lamps.