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In 1930 Philips introduced its first ultraviolet lamp for health care, the Ultrasol 5013. It emerged from the company's research into electronic tubes and was based on a full wave mercury rectifier with UV-transmissive Uviol glass. In 1931-32 it was replaced by this Ultrasol 5023, and like General Electric's American S-2 Sunlamp of similar power it combines a mercury arc with the therapeutic benefits of infrared heat.
At the heart of the lamp is spherical discharge vessel having a pair of offset triple-coil thermionic cathodes, between which an arc-shaped discharge bows upwards such that it is centred within the bulb. A sizeable quantity of mercury is present, the heat of a tungsten filament in the outer bulb causing a large vapour pressure to develop. The filament also acts as an electrical ballast for the discharge.
The spherical arc tube was chosen to elimitate cold crevices where mercury could condense, thereby raising its vapour pressure which is important to increase its efficiency of UV generation. Note the unusual glass-to-metal seals, which each consist of a pair of chrome-iron plates sealed into the sidewall of the glass sphere. An increased arc length is beneficial to raise its voltage and ensure that proportionally more of the mains voltage is dissipated in the discharge rather than the series filament, but too long an electrode gap would raise the ignition voltage above the mains level. This was partly overcome by opting for the spherical tube with short electrode gap, whose arc bows upwards and raises its length and voltage only when hot. Nevertheless, radiometric measurements reveal that this lamp is very inefficient, with about ten times less erythemal power than GE's S-2. As such it was rather quickly discontinued. |