the Sun-Kraft type A1 sunlamp was fitted with a contactless mercury vapour glow discharge tube
Sun-Kraft type A1 sunlamp (left) with
detail of its mercury pool (right)
Ballasting coils can be kept small by increasing the frequency above the standardised
line frequencies of 50 or 60 Hz. By operating a gas discharge lamp at this higher
frequency, the desired current limiting effect can be reached with a much smaller and
cheaper coil. For this reason tube light illumination operating at a frequency of about
400 Hz can be found in ships, aeroplanes and in industrial plants. Frequency
conversion for domestic use is uncommon and discharge tubes in sunlamps were
therefore mostly operated at the standard lower line frequencies.
A special form of high frequency
operation had been applied though in
sunlamps from Sun-Kraft, like their type
A1 on display here. Sun-Kraft specialised
themselves in contactless mercury
vapour glow discharge tubes. In these
tubes the glow discharge was enforced
by placing the tubes in a high frequency
electromagnetic field, generated by an
electronic oscillator. Due to the lack of
electrodes the lifetime of these tubes
was virtually unlimited as long as they
were not damaged mechanically.