Low-Pressure Mercury Arc for Ultraviolet Calibration

A study was made of the visible-ultraviolet irradiance produced by a commercial low-pressure mercury arc operated by an accompanying power supply with a rated input of 115 volts ac. The 2537 Å irradiance was 3.9 μW/cm2 at one meter at a specified axial position in ambient air at 20°C. The 2537 Å irradiance changed less than 2% when power-supply input varied from 105 to 130 volts ac, with environmental temperatures of 19.4°C and 26.8°C and ambient air flows of 11.2 and 14.7 cm/sec, respectively. Spectral analysis of the lamp irradiance in the 1900–6000 Å range showed that 92% originated from the 2537 Å mercury resonance line while the remaining 8% is attributable to 12 other mercury lines. The intensities of 55 other spectral lines of negligible intensity in the investigated range are given relative to the intensity of the 2537 Å line. The experimental mean free path of electrons in mercury vapor at 1 mm of Hg and 273°K was ~5 × 10−3 cm whereas kinetic theory gave 6.7 × 10−3 cm.