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On the Antenna-Earth Current during Thunderstorm
For the measuring of the sudden field change during thunderstorm, ”Antenna-Earth Current Method” is often used because of its simplicity. In this paper, the causes which disturb this measurement are mentioned.
Antenna is a lead wire of about 10m long, stretched horizontally at a height of a few meter.
It is earthed through a galvanometer. The current measured in this way is proportional to the variability of the field, so that we can obtain the quantity of sudden field change due to lighting discharge from the ballistic throw of galvanometer.
It must be noted, however, that rapid field recovery which follows the lightning discharge decreases the ballistic throw. This decrease attains considerable amount, and in this case, the photographic record of this current takes the oscillation type. By the way, we can find another type “creeping”, while the galvanometer is setting in a state of dead beat motion.
When the potential gradient increases abnormally, “point-discharge” takes place at the antenna wire, and disturbs this observation. If the potential gradient increases to such magnitude by lightning discharge, point discharging current flows through the galvanometer, superposing the current induced by the field change, and makes the galvanometer's motion ”creeping”
In conclusion, photographic records of the antenna-earth current by lightning discharge take the 3 types, critical aperiodic motion, oscillation and creeping. In the first case, we can find readily the quantity of sudden field change; in the second case, we must take the decrement of ballistic throw in consideration; and in the last case, we can't deduce the field change.
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