1.13 The Magnetic Field

The magnetic field that exists around a current is called an H field. This field is a force field that reacts against another magnetic field. The force field can be observed using a small compass or by noting the alignment of iron filings on a piece of paper that is threaded by a current carrying conductor. The Biot Savart's (Ampere's) law says that the line integral of H around any closed path is 4πI, where I is the current threading that loop. The value of H along the path of integration depends on permeability. In a high permeability part of the path, H is a low intensity field.

The H field is a vector field as it has intensity and direction at all points in space. The shape of the H field is usually represented by a series of curved lines. The H field intensity is the greatest where these curved lines concentrate. For a single round conductor carrying a steady current, the H field is represented by circles that surround the conductor (see Fig. 1.3 in Section 1.18).

Figure 1.3 Poynting's vector for a pair of conductors carrying energy. E and H field vectors exist at all points in space. P = E × H. Note that P shows the density and direction of power flow at every point in space.


The second description of the magnetic field is the induction or B field. The induction measure of the field is needed whenever voltages are involved. The value of B in teslas ...

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