Observation of Variability of Earht's Vertical Magnetic Field and Some Considerations on the Effective Magnetic Field due to Electric Works
Imamiti, S.
Abstract
At the Kakioka magnetic observatory, induction current in a dingle turned horizontal circuit caused by changes of the vertical magnetic field of the earth is recorded continuously. The area of the circuit is 1 x 10５ square metres and it is buried about 20 cm below the earth's surface to prevent any current due to wind and other artificial causes. In the circuit a galvanometer is connected in series, which has a sensibility of 3.2 x 10-8 amp./mm. or 6.4 x 10-7 volt/mm and a period of 3 seconds. Induction current i in this installation is given by the formula i = 0.5Δｚ/Δt amp. Variability correspond to bay type variations, pulsations, initial part of magnetic storms and magnetic variations accompanied by Dellinger effect are studied. But they are so disturbed by artificial magnetic field that it is impossible to obtain exact conclusions except the following points; (1) there are no special changes in the variations above mentioned. (2) on disturbed days much minute oscillations of period 3-5 seconds and of amplitude 0.02-0.05γ are recorded than on calm days. (3) bay type variations and pulsations are generally accompanied by such fine oscillations. The artificial magnetic field are caused mainly by the Tobu Electric Rail Way and that of the government in Tokyo and in it's suburb. Especially the Nikko line of the former, which is about 50 km westerly away from Kakioka gives much effect than others. It is a noticeable fact that the mean diurnal variation of variability at Kakioka is similar to the mean load curve recorded at a transforming station of the Tobu-Nikko line, and curves corresponding to time derivatives of current-time curves for electric cars of the same line are found in the records of variability. Strength of the magnetic field due to a rectangular circuit formed by a positive feeder and a rail is calculated, and it leads to the result that the vertical field at a point 50 km. away from the circuit gives no detectable effect to the above installation. If we take, however, into consideration of leakage current from the rail to the ground, the rectangular circuit is replaced by a straight current of which strength is equal to the difference between the current flowing in the feeder and that in the rail. In this case, under some conditions of the underground, this straight current, which is 10km. of length and 300 amp. of strength will give appreciable effect at the point.
Magnetic effects of leakage currents alone are also calculated. They are treated as circular horizontal currents of x２ +ｙ２ +２μｙ - ｋ２＝０ and the current density is assumed to be inversely proportional to the distance from x axis, where μ is the variable parameter and k is a constant. Calculation shows that the vertical component of magnetic field at any point due to such circular currents is mostly caused by the currents with which the point is enclosed, and the thickness of this current sheet to give a fairly great deviation as 10mm on a recording photographic paper is about 600 meters. the current density at the observing station corresponding to the above deviation is 1 x 10-１２ amp, which is obtained from a recorded earth current. It must be remembered that there are no detectable artificial magnetic effect on the ordinary magnetic observation at Kakioka.
