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タイトル: 32.電磁式地震計の磁気回路について
その他のタイトル: 32. Note on the Magnetic Circuit of the Moving-coil Type Seismometer
著者: 嶋, 悦三
著者(別言語): Shima, Etsuzo
発行日: 1961年1月10日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第38冊第4号, 1961.1.10, pp. 545-557
抄録: It is aimed in this paper to contribute to the design of a highly sensitive seismometer. We know that the voltage sensitivity A is a product of B and I, where B is the flux density of the air gap of the magnet used and I is the effective length of the conductor. So we can easily obtain the desired aim by providing the seismometer with a powerful magnet of high flux density in its air gap. Thus the design of the magnetic circuit is one of the most important items at the present time. It is well known that analyses of magnetic circuits and electrical circuits can be similarly reached, if we consider the analogous relations between them. Namely, magnetomotive forces, magnetic fluxes and magnetic resistances are analogous to electro-motive forces, electric currents and electric resistances respectively. In the case of the electrical circuit, the difference in the resistivity of the conductor and the insulator is so large that no current will leak out of the circuit. In the case of the magnetic circuit, on the contrary, the ratio of the permeability between the conductor and the insulator is found to be as low as 103-104, so we have to consider the effect of the leakage flux. This is, notably one of the reasons why the design of the magnetic circuit is somewhat complicated as compared with that of the electrical circuit. The behaviour of the leakage flux is, in fact, so complex that the accurate evaluation of the leakage flux is almost impossible. So we are compelled to estimate the leakage flux by assuming an approximate magnetic path. At present, two approximate methods are in a practical way used for this purpose. The first method is more accurate though it is rather involved. In this method, first we draw the lines of magnetic force and of equi-magnetic potential so that .the permeances, that is the reciprocal of the magnetic resistances, of the part bound by these lines are equal; and second we evaluate the total permeance of whole space considering that these permeances of the elementary portions are each connected with the other three dimensionally. The weak point of this method is in the troublesome calculation of the permeances of the circuit. On the contrary, the second method is more practical and easier than the above mentioned method. In this method, we assume that the shape of the path of the magnetic flux can be approximated by a circular path. Then the total permeance of the circuit is evaluated easily bearing in mind that the each part of the circuit is connected in series or parallel with the others. We have felt it necessary, in our study of the theoretical and experimental approach to the design of an electro-magnetic seismograph, to know whether the above-mentioned simplified assumption can be used in the analysis of the magnetic circuit of the actual seismometer or not, because, no relevant study of the magnetic circuit used in ordinary seismometers is known to the writer. And so the main purpose of this paper is to check this method, and to investigate how to achieve a large useful flux. In section 2, is given a brief review of the theory pertinent to a design for a magnetic circuit. Special considerations were given to obtaining optimum working point of the magnet with a minimum volume of magnetic material. In section 3, an example is set out taking a model circuit as shown in Fig. 1. For convenience we divide the magnetic circuit into seven parts. We could verify in some actual circuits, that the total permeances can be calculated by the combinations of the above-mentioned seven parts, while in others some slight modifications to the path are necessary. In section 4, we examined the magnetic circuits of eight bore-hole-seismometers, and found strong agreement between the values calculated by the second method and the direct measurements of the useful fluxes. We may therefore conclude that, the second approximate method can be used safely in analysing the magnetic circuits of the seismometers.
URI: http://hdl.handle.net/2261/12001
ISSN: 00408972


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