Observations of electric currents flowing in the flying cables of barrage balloons were made during an investigation of the problem of protecting balloons in thundery weather. I t was found th a t charges of the order of coulombs are transferred between balloon cable and air by currents ranging from milliamperes to kiloamperes. The observations give some indications of the distribution of charges in clouds.
I n t r o d u c t i o nKites have been associated with the study of atmospheric electricity since the days of Franklin, and the risks involved in handling them in thundery weather have been appreciated since Richmann was killed when repeating Franklin's experiments. During the period 1939 to 1945, kite balloons were used for defence purposes on a large scale, and an investigation of the problems of protecting balloons and crews was begun in 1937. At this time the belief was held th at many balloons had been destroyed through the agency of 'brush discharge', and laboratory experiments in which wet balloon fabrics were set on fire by the passage of a current of the order of 1 mA/cm. width supported the view. The investigation therefore included measure ments down to low values of the current flowing in the balloon cable in order to study the relation between atmospheric conditions, balloon current, and risk of destruction or damage.Measurements were made at the Balloon Development Establishment, Cardington, with indicating instruments, cathode-ray oscillographs, and magnetic indicators, and in operational balloon barrages with magnetic indicators. The data obtained from widely scattered barrages indicated that destruction of balloons by brush discharge, if existent, was of minor importance, and th at in the large majority of cases damage or destruction could be attributed to lightning strokes involving currents of hundreds of amperes rather than fractions of an ampere.Some degree of protection against lightning strokes was attempted by mounting a spike 3 m. long above the nose of the balloon. Comparative magnetic indicator measurements suggested th at this protection reduced losses through lightning by a factor of 3 or 4. The highest recorded stroke current survived by a protected balloon was 30,000 amp. Complete protection against lightning strokes could not be hoped for, as two occasions are known on which the flying cable was ' demetal lized'-the vaporization of the steel occurring so quickly th at the hemp core was not scorched.The investigation was carried out as an urgent service requirement with the equipment available, and not as a scientific experiment. In particular, the balloons in barrages were of necessity flown entirely according to operational requirements. Although the information obtained was adequate for the object in view, more [ 304 ] on May 11, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from
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