Continuous current control, so familiar in the operation of high-vacuum tubes, has not been possible, except under special circumstances, for gas tubes. Even current interruption has been awkward, except for low currents, for the usual manner of interrupting the current is to decrease the anode potential to zero. The time to switch off the gas tube has been of the order of the deionization time for the gas employed. Now a method is developed for switching off mercury-pool arcs using a third electrode. There is no interference with the main power circuit and, in fact, the potential on the anode causing the electric field aids the dispersal of the charge carriers when the arc has been interrupted. The switching-off time is much decreased because this anode-to-cathode voltage sweeps all the charge carriers out of the tube. Switching off is effected by passing a reverse current of equal or greater magnitude than the arc cathode current through the tube for a time long enough to interrupt the cathode spot. A technical difficulty arises in that the third electrode introducing the reverse current has to have an already formed or an easily formed cathode spot since this third electrode is a cold cathode. Many methods for forming the cathode spot are discussed. The method finally used is probably not the best one but it has the virtue of being easily effected. There appears to be no limit as to the current that can be interrupted if the spot-forming mechanism is altered. Energy used in not an important factor. The amount varies with the time to switch off and does not influence the actual switching process. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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Call number: LD2668 .T4 1957 T82 / Master of Science

The present work was performed at Sherwin–Williams Sweden group AB with the objective of comparing short-wavelength light emitting diodes (UVB/UVC) with the conventional mercury arc lamp as a curing method of acrylate-based, UV-paint undergoing free-radical polymerization when exposed to UV-radiation. Due to environmental and health risks, mercury-doped radiation sources will be phased out in the near future, according to the United Nations Minamata convention, hence new alternatives are needed. Light-emitting diodes differ from the mercury arc lamp as they provide semi-discrete output intensity lines within the UV spectrum instead of a broad output distribution with several main intensity lines. The power output is also considerably lower compared to the conventional method which limits the irradiance and dose that are key parameters in activating and propagating free-radical polymerization of UV-paint. Seven different light-emitting diodes between 260–320 nm was examinedand compared to the conventional mercury arc lamp. Cured coatings were evaluated by measuring the relative extent of acrylate conversion with ATR-FTIR and micro-hardness indentation test. Both methods correlate to the relative cross-linking density and qualitatively describe the curing process for each radiant source at a specific irradiance and dose. Three different paint formulations with widely different properties were used in the experiments. All three paints were able to cure with one or several light emitting diodes at comparable doses and 10 to 20 times lower irradiance to the conventional mercury arc lamp, resulting in similar acrylate conversion and hardness.