As a gaussmeter measures the magnetic
flux density nearly at a point, a flux meter measures the total
magnetic flux across an area (that is, the integral of flux density
over an area or, for constant flux density, the flux density times
the area).
It is more accurate, however, to say
that what is measured is not flux, but the change of flux. According
to Faraday’s law of induction,
E = N dφ/dt
Where E = voltage across a coil of n
turns
φ = iTEGRALB dA = flux (2.60)
B = magnetic flux density
A = area
N = number of turns on the coil linked
by flux φ
Integrating this equation,
φ = (1/N) iNTEGRAL (E dt + φ0)
Where φ0 is an arbitrary constant of
integration, normally set to zero to begin the measurement.
That is to say, it is possible to
measure an amount of flux passing through an object such as a magnet
by placing the object to be measured in a tight-fitting coil, then
removing the object, while integrating the voltage across the coil
with time.
This is the principle of the fluxmeter.
Alternately, it is possible to remove the object, rotate it end for
end 180°, and reinsert it into the coil. In this case the change of
flux is twice that through the object.
The sensor for a fluxmeter is just a
coil of wire, usually made at the time by the operator. The wire may
usually be of small diameter, because very little current flows
during the measurement.
The larger the number of turns, the
larger the signal. However, if the coil resistance becomes
relatively\ high (possibly 50 Ω or more), some flux meters with
relatively low input resistance may require a correction.
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