R
Electric field meter
1
PURPOSE AND CHARACTERISTICS
The electric field meter 11500.10 (referred to below for
brevity as the EFM) is used for investigation of the field
strength in electrostatic fields.
Furthermore, in conjunction with the voltage adapter of the
EFM, static measurement of direct voltages is possible and,
remarkably, this also applies to relatively low voltages:
measuring ranges 10/100/1000 V DC.
The handy instrument, mounted on a rod, requires a DC
supply between 14 and 18 V.
The unit has a data interface which can be connected to the
9 pole D-connector of a serial RS232 interface of a
computer by means of the supplied cable.
2
MEASUREMENT PRINCIPLE
The measurement principle of the EFM is based on the fact
that an electric field induces charges at the surface of a
conductor, the superficial charge density
σ
of which is
proportional to the normal component
E
n
of the field
strength
E
prevailing at the conductor surface:
σ
=
ε
o
·
E
n
; (
ε
o
= 8,86 · 10
12
As/Vm).
Since the electrostatic field at conductor surfaces is always
perpendicular to the surface,
E
n
=
E
.
A gold-plated flat electrode with six radial sectors is used as
the probe of the EFM. For measurement purposes, the
electric flux to the probe is chopped periodically by means
of a vane wheel of appropriate shape, rotating closely in
front of the measurement electrode and electrically
connected to equipment earth. Depending on the position of
the wheel, the electric flux to the probe is screened off to a
greater or lesser extent and therefore fluctuates periodically
between its maximum value and zero.
The measuring electrode is connected to equipment earth
through a high resistive load. The variation of the induced
charge produces periodic voltage pulses, the amplitude of
which is proportional to the electric field strength
E
n
. Pulses
are fed to an electronic circuit, which delivers a DC voltage
output proportional to the pulse amplitude and thus to the
field strength
E
n
prevailing at the measurement electrode.
Any conductor brought into an electric field modifies the
original field to a certain extent; this also applies for the
EFM. In a given measurement arrangement, the field
strength
E
prevailing at the measuring head is proportional
to
E
o
, the original field strength:
E
o
=
kE
.
If the geometry between the measuring head and the
conductors producing the field is maintained, comparative
measurements can be carried out without any adverse
effects. But measurements series in which this geometry is
varied to a certain extent are also possible, provided that
the factor
k
remains sufficiently constant.
Calibration of the instrument in the works takes place in a
special calibration jig; it consists of a plate capacitor, one
plate of which has a circular cut-out in the centre, in which
the measuring head of the EFM is embedded flush with the
plate. In this arrangement, the indication of the measured
value is so adjusted that it corresponds to the calculable
field strength
E = U/d
for a plate capacitor with voltage
U
and plate spacing d. In a plate capacitor therefore, the
undistorted field is measured directly by the EFM, i.e.
k
= 1
and
E = E
o
; the distance between the plates must then be
d
= 100 mm. Such a capacitator arrangement can be set up
in an experiment by screwing the capacitor plate 11500.01,
which has a central hole, onto the EFM and using the
capacitor plate 06233.01 of the same size as the
opposing plate (Fig 2).
For the electrostatic voltage measurements, a capacitor
plate mounted on an insulating support in the voltage
adapter is placed at a defined distance in front of the
measuring head. The field strength measured by the EFM
is then a measure of the voltage applied to the capacitor.
PHYWE SYSTEME GMBH · Robert-Bosch-Breite 10 · D-37079 · Göttingen · Telefon (05 51) 6 04-0 ·Telefax (05 51) 60 41 07
11500.10
Operating instructions
The unit complies
with the corresponding
EC guidelines.
