George S. Forbes

Spectral Energy Characteristics of the Mercury Vapor Lamp

By means of a large aperture quartz spectroradiometer the spectral energy distribution of a special variable-length mercury lamp was measured between 14,000 A and 2300 A under widely varying conditions. The effects of current, voltage and ventilation on the radiation at fifteen important maxima were measured, including three maxima in the infrared, four in the visible, and eight in the ultraviolet. The composition of each maximum is given, and curves are plotted showing the degree of resolution used; the variation in intensity of each maximum with voltage gradient at constant current; the variation of intensity of the strong ultraviolet maximum near 3660 A with voltage for five current values from two to four amperes; the stationary characteristic curves of a high pressure arc; two typical cases of spectral energy distribution at constant power; and the variation with current of the intensity of each maximum at constant power.It was found that for constant current the energy in each maximum except those in the infrared increased linearly with the voltage after a certain minimum (near 8 volts for 2.5 amperes) was reached. The pressure was measured for each stationary state used, the extreme values being 20 mm of mercury and two atmospheres. Where power input was constant the energy radiated in each maximum increased rapidly with decreasing current. It was found that the most satisfactory condition for running an arc at pressures under two atmospheres, at least, was to have the pressure as high as possible, so that for a given power input the voltage was high and the current low. This condition gives the greatest efficiency and the least change in energy distribution with total energy variation. The relation of pressure to the other variables and the effects at higher pressures are being investigated further; also an absolute determination is being made of the energy available in each wave-length for photochemical purposes.

Spectral Energy Characteristics of the Constricted Mercury Vapor Lamp—an Extremely Concentrated Source of Ultraviolet Illumination

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.

Relations Involving Internal Pressure, Intensity, Mercury Transfer, Cross-Section, and Electrical Conditions in Mercury Vapor Lamps

Internal pressure was measured in two special quartz mercury arcs developed in previous investigations. Numerous curves present the data. Pressure is a function of cross-section, voltage gradient, and current only, and independent of arc length and the mode of cooling. Above two atmospheres pressure increases about 140 mm for unit change in voltage gradient, irrespective of other variables. In the constricted arc, 2 mm in diameter, where the cooler absorbing vapor shell is very thin, intensity at given current strength is quite closely proportional to pressure from a few millimeters up to five atmospheres.With the addition of thermocouple inlets at the mercury surfaces and calibrated measuring tubes, mercury transfer for equal electrode temperatures was measured in the 9 mm arc. The relations are little changed when the cathode is at least 20° to 40° hotter than the anode. Apparently zero at the lowest pressures and voltage gradients, it rises to five gram atoms per faraday from anode to cathode at 5 volts/cm and remains nearly constant up to 25 volts/cm. Current strength is of secondary importance. These conclusions are amply demonstrated by curves. The relative importance of the electron current and the positive ion current under the various conditions studied is briefly discussed.