12 cHAPTER 2: Background
Model 425 Gaussmeter
The voltage that is generated by the Hall sensor goes through an AC coupled pro-
grammable gain stage and is sent directly to an RMS-to-DC converter. The signal is
then sent into the A/D converter. The digitized data is then sent to the microprocessor.
The monitor output will provide an unfiltered AC voltage proportional to the mea-
sured AC field. Refer to section 4.5.2.2 for the procedure to set the AC wide band
mode.
2.2.3 Monitor Output
The Model 425 has a monitor output that provides an analog representation of the
reading and is corrected for probe offset and nominal sensitivity. This monitor output
makes it possible to view the analog signal, which has not been digitized. The monitor
output can be connected to an oscilloscope or data acquisition system for analysis.
2.3 Flux Density
Overview
2.3.1 What is Flux
Density?
A magnetic field can be envisioned as lines of force measured in maxwells (Mx). In the
cgs system, magnetic flux ( ) is the Mx, where 1 Mx = 1 line of flux. In the SI system,
magnetic flux is the weber (Wb), where: 1 Wb = 10
8
Mx.
Flux density is the number of flux lines passing perpendicular through a plane of unit
area (A). The symbol for flux density is B, where B = /A. The cgs system measures flux
density in gauss (G), where 1 G = 1 Mx/cm
2
. The SI system measures flux density in
tesla (T), where 1 T = 1 Wb/m
2
.
Flux density is important when magnet systems concentrate flux lines into a specific
area like the pole pieces of an electromagnet. Forces generated on current carrying
wires like those in a motor armature are proportional to flux density. Saturation of
magnetic core material is also a function of flux density.
Additional conversion factors can be found in the Appendix.
2.3.2 How Flux Density
(B) Differs from
Magnetic Field
Strength (H)
Flux density is often confused with magnetic field strength. Magnetic field strength is
a measure of the force producing flux lines. The symbol for magnetic field strength is
H. In the cgs system, it is measured in oersteds (Oe). In the SI system, it is measured in
amperes per meter (A/m):
1 Oe = 79.58 A/m
Flux density and magnetic field strength are related by the permeability (µ) of the
magnetic medium. B = µH. Permeability is a measure of how well a material makes a
path for flux lines.
The confusion of flux density and magnetic field strength is also related to permeabil-
ity. In the cgs system, the permeability of air (of vacuum) is 1. Therefore, 1 G = 1 Oe or
B = H in air. Many people incorrectly assume, therefore, that in the cgs system, B = H at
all times. Adding to the confusion, in the SI system, permeability of air is not 1, so B is
not equal to H even in air.
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