After having determined in a previous investigation the spectral energy characteristics of a high intensity quartz mercury arc of 8 mm bore for various conditions of operation, the writers here describe a similar set of experiments with an arc constricted to a tube of 2 mm internal diameter and 38 mm long, thus doing away with the outer absorbing layer of relatively cool mercury vapor. Currents of from one-third to five amperes were used, air cooling being necessary between one and two amperes, and water cooling above this point, except at high pressures, when air cooling was necessary even at low currents. Voltage gradients of from seven volts per cm to 58 volts per cm were used. The pressure in the arc, which was of special design, was measured with a mercury manometer, and varied from 29 mm to 3920 mm. The energy in each of the more important groups of lines was measured with a thermopile, and it was found that this type of arc was of very high efficiency, and of tremendous intensity and convenience for slit illumination. Curves are given showing the spectral energy variation with varying current and voltage, and it is shown that approximately the best combination of intensity, efficiency, convenience, and life is obtained at atmospheric pressure, one ampere current, and 110 volts with suitable series resistance. No decrease in efficiency with increasing voltage was observed at pressures up to five atmospheres. Where extreme intensities are desired, currents up to five amperes may be used, but the life of the lamp is shortened.
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