The Earth inductor is a device designed to measure the strength of the Earth's magnetic field. It works by rotating a coil or wire (the Earth's magnetic field is weak, so the wire must have many turns) about an east – west axis as shown below.
To start with the coil is oriented vertically (above left) and rotated quickly by half a turn. The amount of flux cut is
so the average voltage produced is
The pulse of pulse produced can be measured by a galvanometer hence V found. This pulse will be proportional to the horizontal component of the Earth's magnetic field. Similarly when the coil is oriented horizontally and rotated through half a turn the pulse produced is proportional to the vertical component of the magnetic field. The vertical and horizontal components of the Earth's magnetic field, and the angle of dip can thus be found.