Cylinders of lumite glass 35 inches long by 10¼‐inch diameter (such as are used on gasoline filling‐station pumps) were used for dielectrics, and 0.005‐inch aluminum foil for plates. A 7‐inch glass margin was left at the ends of the cylinders. Each condenser had a capacity of 0.012 microfarad and would withstand steady voltages of over 100 kilovolts without ``flashing over.'' Sets of rings of −inch
copper tubing were made to fit the cylinders at the edges of the plates, both inside and out. The rings provided a ``curled'' or ``rounded'' edge, in place of the rather sharp edge of the foil itself. The effect upon the ``edge corona'' and ``flash‐over voltage,'' due to various‐width bands of aluminum paint on the glass surface adjacent to the metal‐plate edges, was tried, with and without the above‐mentioned rings, in disruptive discharge and steady‐voltage tests. The results showed that the highest voltages could be obtained, without break‐over occurring, when the glass margins were chemically clean. It was also found that the best type of plate‐edge construction was either the plain aluminum‐foil edges without paint, or the curled edges (i.e., with copper rings) with a ¼‐inch band of paint.