142. Measuring the magnetic permeability of iron

The magnetic properties of the substance are characterized by magnetic permeability \(\mu\). If the value m is much greater than one, as in the case of iron, the magnetic permeability can be measured by the phenomenon of electromagnetic induction.

EMF induction is equal to the speed of change of the magnetic flux penetrating the coil. If an iron core is inserted into a long coil, the magnetic induction will increase by \(\mu\) times. Consequently, the magnetic flux and EMF induction will increase by the same factor. When a circuit with a DC source supplying the magnetizing coil opens, an inductive current is generated in a second, small coil located over the main coil (fig. 1), which is detected by the galvanometer. If an iron core is inserted in the coil, the deviation of the arrow of the galvanometer will be \(\mu\) times greater. Measurements show that the magnetic flux increases by a thousand times in some cases when the core is inserted into the coil. Therefore, the magnetic permeation of iron is large.

Since the iron increases the magnetic flux, we can conclude that the magnetic field of the elementary electric currents in the iron core coincides with the magnetic field of the coil magnetizing the iron core. So, the elementary electric currents are oriented in the magnetic field so that their direction coincides with the direction of the current in the coil (fig. 2).