The Search Coil
The search coil is a small flat coil of fin wire consisting of a large number of turns – between a few hundred and several thousand. The coil is mounted on an insulated hadle as shown below.
When the coil is placed near to a region of changing magnetic field an EMF is induced in the coil. The induced EMF is proportional to the rate of change of the magnetic field through the coil according to Lenz's Law,The changing EMF can be displayed on an oscilloscope. Because the rate of change of the magnetic field determines the induced EMF, for a sinusoidally changing current there will be a directly proportional relationship between the induced EMF and the strength of the magnetic field. We can calibrate an oscilloscope therefore to display the magnetic field strength. This is probably best done by switching the time base off so that the EMF is displayed as a vertical line.
The coil can be calibrated with reference to a standard varying magnetic field and any unknown sinusoidal magnetic field of known frewuency then found.