The magnitude of the magnetic field on the axis of a circular current loop  
is given by Eq. 2.If a coil carrying current magnetic field is measured as a function of  
distance, this process is called magnetic mapping.Search coil is a small area coil of closely spaced number of turns, N and  
area, A. If such coil is inserted in a space of varying magnetic field, an induced  
emf is produced.is parallel to A, and A is constant, faraday's equation becomes; 
 emf = -N.A 
dt 
dB 
If the rate of change of B is known, the magnetic field, B, can be measured  
by measuring the induced emf.The field has its largest value in the plane of the loop and decreases as the distance z increases.where ?=???