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August, 1931

U. S.AIR SERVICES

31

complete shielding afforded by metal cabin planes completely eliminates the effect of the induced charge due to the collapse of the electrostatic field. The same applies to Zeppelins.

d. SUDDEN CHANGE IN AIR PRESSURE
The intensity of lightning varies greatly for different strokes. It would seem, however, that the very severe discharge causes a rapid heating of the air and an increase in pressure in the immediate vicinity. The pressure set up where the air is free to expand is not as great as originally supposed. The fact that three balloonists have come through storms where the stroke was within a foot or two of them would indicate that the hazard from this source is not serious, even for the very heaviest discharges where the path can be moved out a few feet from the pilot.

e. SEVERE SOUND OR PRESSURE WAVES
The effect of the severe sound and pressure waves may be responsible for the shock suffered by pilots and others who have been within a few feet of lightning strokes. The duration of the effect seems to vary considerably with the individual, and may cause effects somewhat similar to those suffered from shell shock. Many of the factors producing shell shock are present although there are others in addition. The severity reduces rapidly with distance. It would therefore seem that the point of contact between lightning and plane well away from the pilot will do much to eliminate any serious effects which may cause the pilot to lose even temporary control.
The pilot can, of course, be easily protected from sound or pressure waves by providing a suitable compartment or other sound absorbing equipment.

f. CURRENTS INDUCED IN THE BODY BY AN ELECTRO-MAGNETIC FIELD
A STROKE of lightning may consist of a single discharge or several discharges within a very short space of time. The strength of the electro-magnetic field will vary directly as the current in the dicharge, and inversely as the distance. A current of 400,000 amperes in a stroke will produce a field of 2,620 maxwells at one foot distance; 262 maxwells at 10 feet distances, and 131 maxwells at a distance of 20 feet. 
Any change in the electro-magnetic field passing through either high or low resistance material will induce a voltage and current. The induced voltage and current will depend upon the rate of change in the magnetic field. It is evident that the lines of force passing through a person will induce currents, the action being similar to that in a high frequency furnace. Several strokes in rapid succession may induce a greater current or potential than a single stroke of higher maximum magnitude but having a slower rate of change.
While it is possible to screen the electrostatic field, it is practically impossible to effectively screen the electro-magnetic field so as to prevent the lines of force passing through objects in the vicinity of the field. The field may be set up by the current of a portion of the discharge in the air, or in a conductor in the aircraft. It would seem that the heaviest discharges taking place close to a person may induce enough potential and current to affect the system, or at least to paralyze the nervous system temporarily.
A study of the factors affecting the induced potential and current go to show that the electro-magnetic field may be materially reduced by removing the shunt path from the immediate vicinity, and by forming multiple paths around the pilot or passengers so that the field set up by the current in one path tends to neutralize the field set up in the other. The fact that some people have not been affected although within a few feet of the discharge, would indicate that a reduction of the field strength by an appreciable amount would entirely change conditions and provide protection even for the most severe strokes.
Further investigation may show that the induced current does not constitute a hazard. The magnitude of the voltage generated and the current induced for heavy strokes in the immediate vicinity, however, would indicate that the possibility of affecting at least the nervous system cannot be ignored without very definite proof to the contrary.
Although little has been accomplished in determining whether or not the shock or stunning effect is primarily due to electrical causes or to physical conditions similar to those producing shell shock, the same method will improve conditions for either case- which consists primarily in initiating the point of contact to the stoke as far away from the pilot as possible.

g. HAZARD DUE TO THE EFFECT OF INTENSE LIGHT UPON THE PILOT
The light from a stroke particularly at night may have the effect of blinding the pilot when passing through the line of vision. At night the iris is wide open, and it is possible that the ultra-violet light might have some effect upon the pilot for a discharge striking the nose of the plane. In most planes, however, the discharge would be to one side and above or below the direct line of vision. It is believed that the hazard from this cause is small. While intense light may have the effect of blinding the pilot for a short time, any serious effect due to ultra-violet light may be eliminated by the use of glass in the windows or goggles which would absorb any injuries rays.
While the effect of lightning upon the pilot or passengers undoubtedly causes the greatest and most uncertain hazards, there are other hazards to the plane or aircraft which need careful consideration.

1. FIRE HAZARD DUE TO IGNITION OF COMBUSTIBLE MATERIAL USED IN THE CONSTRUCTION OF THE PLANE
The fire hazard to the metal plane is negligible although intense heat exists at the point of contact with the lightning and the plane. The fire hazard is practically negligible even where material which will support combustion is used. Where an inflammable covering is used to contact with metal, the fire hazard is apparently negligible. Inflammable material which is readily ignited may be set on fire by the crater. The severe rush of air following the stroke has the effect of extinguishing the flame, the discharge being in the nature of an explosion. The rapid expansion of the gases even in the fabric apparently absorbs sufficient heat, which together with the blast of air following the discharge prevents ignition. This is particularly true where a fabric is used to cover duralumin or other metal. The heat conduction of the metal and the low resistance of the path afforded, tend not only to absorb the heat but to reduce the energy dissipated.
A rather severe series of tests run on a model Zeppelin covered with fabric showed that it was impossible to ignite