ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 9)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part #H-600042 9
Electric fields are measured through the employment of a
displacement current sensor. A displacement current sensor
operates on the principle that two parallel conductive flat-plate
electrodes, when electrically connected together, will exhibit a
displacement current that flows between the two plates when
immersed in an electric field. This can be visualized by
remembering that the electric field between two such plates must
be zero when they are connected together; i.e., because they
are at the same potential there can be no electric field between
them (an electric field exists when the potential on the two
electrodes is different). Another way of viewing this phenomenon
is to understand that when immersed in an electric field, the
external field causes a redistribution of electric charge on the two
electrodes and this redistribution of charge is in reality just a flow
of current, a displacement current, between the two plates.
The HI-3603 uses this principle to detect electric fields by
measuring the displacement current caused by the ambient field
between two closely spaced circular disks. By placing such a
detector in a known electric field, the displacement current can
be related directly to the magnitude of the field causing it,
permitting its calibration. A circular sensing plate surrounded by
a "guard ring" is used in the HI-3603 and the displacement
current developed between this smaller diameter disk and a
closely spaced eight inch circular disk electrode is sensed and
converted to equivalent electric field strength. Because the larger
electrode is used as a reference in the measurement process,
for accurate measurements of electric fields, the sensor must be
oriented in such a way that incident field lines strike the smaller
disk. To aid the user in this, the sensing paddle probe is clearly
illustrated, showing the normal orientation for measurement of
both electric and magnetic fields emerging from VDT's.
Magnetic fields are measured through the use of a three-turn
loop wound about the periphery of the circular electric field
sensing electrodes. This loop is shielded from electrostatic fields,
insuring that its response is due solely to the magnetic field.
Open circuited loops are frequency sensitive devices that
provide an output that is proportional to the time rate of change
of the magnetic field fluxing through the aperture of the loop. In
contrast to this, in the case of the HI-3603 design, the loop