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AIR COMMERCE BULLETIN 33 tricity and known as ejectrostatica. In this froup, there is first mentioned a possibil- fy due to the nature of the materiats employed. In the older of cell fabric, contain- Ing a rubberized element, ft was apparently possible to create a static spark by tearing the fabric. The cell fabrie used in the Hindenburg, as far as we cou'ld learn, did net include material possessing thia char- acteristic. Since virtually all of the cells Were copsumed by the fire, no test could Be made of the cell fabric. . ’ The two bungeeg in the stern of the ship connected to the horjzontal membera of the tell, contained some rubber, but ag fur as we know the bungees had not been damaged until after the fire had broken ou Before proceeding further withthe sub- dect of electrostatics, 1t 19 to be remarked that an airship as a body te regarded as carrying an electric charge, the nature and extent of which depend upon the circum- stances. In motion it may aceumulete a charge either through friction with the air or perbaps by mesns of charged water drops such as may be found in clowds or mist. It may accumulate a charge of either posl- tive or negative sign, Thunder clouds may carry a positive or a negative sign. Ac. cording to the evidence in this instance, the ship is assumed to have carried a post- tive charge on Ita outer surface, which ja & semiconductor. This phenomenon is due to the fact that an airship in flight ia within the atmosphere which is electrified. A few of the more interesting features of this befomenon are; that the earth ordinarily a charged negatively; that in the’ atmos- phere there is an electrical field measured in volts per meter (potential gradtenti which in fine weather amounts ta 100 volts per meter, becoming higher as the weather ows Inore disturbed; that the tendency 8 for an equalization current to pasa from the atmosphere to the ground; thit the electrical conductivity of the atmosphere is greater when the atmosphere is humid. Other facts and assumptions are that the total outer surface of the ship haa a uniform potentia'; that the electrostatic effects on the outside of the sbip are separate and apart from those on the inside: that a number of conditions tend to equalize the potentia! of the ship with the surrounding atmosphere; among these is the dissipatian created by the exhaust gases and by the movement of propellers, the edges of the latter being metallcally connected with th: ship's structure; that the landing ropes would serve as conductors of the ship's charge and equalize the potential of the ship with that of the ground, When the ship is held by the landing ropes the elec- trostatic picture 1s such that the surface of the ship after a brief interval, so to gpenk, becomes a piece of the ground e'e- vated Into the atmosphere. The potential differences measured ver- tleally to the earth are called the potential gradient. This gradient is higher over those ereas of the ship where the edges or points project into the atmosphere, espectally over he bow and stern of the rhip. It maw he tncrensed in the presence of charged clouds. The principal protection agninst an elec- trostatle discharge which might serve to ignite an inflammable mixture in or_about the sh'p is the bonding of the ship, Bri fiy such bonding is the connecting up of the many patts of the ship so that electrically it becomes one complete metallic whole. A possible test of the state of this bonding could have been made by detecting throug’ the radio receivers the characteristic noise associated with interference created by im- perfect bonding. In the present instance, ee had been noted, the receiving system of the ship did not give indication that any injury ad occurred to the ship's bonding prior to the accident. We have also considered the possibility that due to a discharge between parts of the ship having different potentials, a spark might have been created. Whether such 4 discharge occurred we cannot say. <Ac- ‘cording to the testimony, tbe ship was bond in keeping with the best known practice, There was one fixture of the sbip in this reapect that received more than pass- ing notice—the unbonded electric wires at the stern_ electric Jamp of the airship, Witness Dieckmann indicated that there might have been a static charge produced by this tail light wiring at the light bulb since the wirlag within it was the only part of the ship which did not have the game potential ag the remaining surface of the whip, a very singll difference, however. Whether such a small electrostatic capacity as the lamp terminal would have been able to produce a spark is highly questionable. Another reason to regard it as improbable ig that no one reported having seen the origin of the fre at the extreme rear end of the ship. BALL LIGHTNING A reading of the record reveals that some space is given to another manifests- tiun of electrostatic discharge; namely, to the possibility that ball lightning might have accounted for the ignition of the mixture, Ball lightning fs supposed to be one of the peculiar species of lightning discharges that bave been observed from time ta time. One of itg features ig that lke a drop of oil on water it spreads and splits into segments, some of which seginents continue for a distance along objects on which ther alight. Although some authorities have dis- clatmed the existence of ball Nghtning, we have considered the idea for what tt might be worth. It does not very well explain the slow burping that some of the witnesse< described as having taken placr at the be- ginning of the action. Moreover, the theory as applied in the present instance would appear to have little substance ginve no one testified to having observed any form of Mehtning, For the same reason any o.her claim made on the ground of lightning as a cause would also seem to fail, because none of the witnesses who testified stated that they observed any lightning flashes in the vieinity of the ship or heard an accom- pacying clap of thunder at the time of the accident, B&U8H DISCHARGE, OR ST. ELMO'S FIRE In order to develop the next possibility to be considered, viz. ignition due to brush diacharge, or St. Elmo's fire, a few addi- tional remarks are necessary upon the subject of electrostatics and the eonditions that actually prevailed at the time and place of the accident, Tt will be recalled that the bow port trail ropes first made contact with the ex- tremely wet ground, 4 minutés before tlie fire. hen they left the ship they ap- peared to be quite dry as dust was observed to fly from them as they descended. These ropes were made of hemp, The atmosphere at the time and place of landing was humid and the ship had abserbed moisture. Tt wes, therefore. reasonable to suppose that in the interval the ropes continued 10 ab- sorb moisture and their conductive qualities increased. Therefore, their contact with the gfound under the circumstances would dis-