We discuss the influence of two factors on the deviations from the model of the magnetic field of a long straight wire. Firstly, we derive the magnetostatic analogue of McCullagh’s formula for the case of the infinitely long straight conductor with an arbitrary cross-section. Using this equation on the basis of its practical significance, we find that the greatest deviations from the long straight wire model should be observed for a system of two parallel and infinitely long straight wires, equal in absolute value and direction currents. In this case the extremal direction of the magnetic field deviations respectively passes through these wires or is perpendicular to the plane passing through them. Our numerical estimations show that any infinitely long straight conductor with a cross-section having dihedral group symmetry can be considered with reasonable accuracy (the relative error in the determination of the magnetic flux density is less than 5%) as the infinite straight wir...

中文翻译：

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关于长直导线磁场模型的适用范围

我们讨论了两个因素对长直导线磁场模型偏差的影响。首先，对于具有任意横截面的无限长直导体的情况，我们推导了McCullagh公式的静磁类似物。基于其实际意义，使用该方程式，我们发现对于两条平行且无限长的直导线，其绝对值和方向电流相等的系统，应观察到与长直导线模型的最大偏差。在这种情况下，磁场偏差的极值方向分别穿过这些导线或垂直于穿过它们的平面。