Electronic Fluxmeter EF 14
5 Operating Information
The magnetic flux
can be represented by the number of the magnetic field lines that is
penetrating a defined cross-section A of a magnetic field,
A
AΒ
0
d
,
where B is the magnetic flux density. In a homogeneous magnetic field it is
= B · A.
The flux cannot be measured directly but must be determined using the electric voltage to
which it is coupled through one of Maxwell’s equations.
When the flux changes, the electric voltage induced in a coil surrounding the magnetic flux is
N(t)u
d
,
where N is the number of turns of the measuring coil. From this follows by integration
2
1
0
d
1
t
t
t(t)u
N
.
Accordingly only the flux change occurring between t
1
and t
2
can be measured. The
measurement is tied on integration and thus the measuring instrument, the fluxmeter, is an
integrator.
The integration constant
0
becomes zero, when the Reset key is pressed in DC operation. In
AC operation the integrator is continuously discharged, a Reset is therefore not required.
5.1 Input Resistance and Coil Resistance
The electrical resistance of the measuring coil is connected in series with the input resistance of
the fluxmeter. The output voltage u
a
of the integrating amplifier depends on the total resistance
R,
ttu
tu d)(
1
)(
a
.
R is the sum of the input resistance of the fluxmeter and the coil resistance.
In the past simple fluxmeters often had an input resistance of few hundred or thousand ohms,
which sometimes even depended on the measuring range. With these fluxmeters it always was
necessary to take the resistance of the measuring coil into account. For every coil and every
range an individual correction factor had to be calculated.
Modern fluxmeters have an input resistance of 10 kΩ or 100 kΩ that is independent of the
range. The input resistance of the EF 14 is 100 kΩ. The resistance of many measuring coils is
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