Snapshot 1: a five-turn coil is placed upright to the field of 0.3 MHz and is shorted
Snapshot 2: a five-turn coil is connected to 1M
–100pF detection circuit for the field of 0.3 MHz
Snapshot 3: an increased-turn (30) coil resonates with the capacitor of 1M
–100pF detection circuit for the 0.3MHz field
The sinusoidal field
induces the electromotive force (EMF)
in a circular coil of radius
being the angle between field and coil axis. Assuming the circular section of coil has diameter
, the coil's inductance is approximated by
. Then the current and voltage in the detection circuit are calculated by
is the impedance of the detection circuit. For the open circuit,
is set at 1 M
and 100 pF; for the shorted circuit, it is 50
The coil current produces a magnetic field superimposed on the original field. The additional magnetic field for the ring current involves Bessel functions. The magnetic field pattern is readily obtained, specifying the time and phase values.
Among the four detection circuits, the detected current increases most generally in this order: open circuit, 1 M
//100 pF, 50
, and shorted conditions. The detected voltage tends to vary in the opposite direction. However, in the detection circuit of 1 M
//100 pF, an extraordinary large current and voltage can occur during resonance between the coil inductance and the capacitor in the detection circuit, as shown in snapshot 3.
 J. D. Jackson, Classical Electrodynamic
s, 3rd ed., New York: John Wiley & Sons, 1998.