CHAPTER 4 MEASUREMENT CONSIDERATIONS
4-5
Range Switch Position
Input Impedance
Maximum Recommended
Coil Resistance
10
6
1000
Ω
≈
100
Ω
10
7
11 k
Ω
≈
1100
Ω
10
8
101 k
Ω
≈
10.1 k
Ω
Table 4.6.1 Model 2130 Input Impedance
4.6.3 Coil Area
When a search coil is used in an area of uniform flux density, the signal strength is directly
proportional to the cross sectional area of the coil. Therefore using a search coil with a larger
cross sectional area allows the Model 2130 to be set to a higher range. The size of the search
coil will often be a compromise between the signal strength and the spatial resolution that is
required. If the area of the coil is accurately known, flux density may be measured in addition
to total flux.
4.7 Selecting a Helmholtz Coil
The Helmholtz coil is actually a pair of coaxial coils mounted such that the distance between
the coils is equal to the radius of either coil. This arrangement allows the measurement of the
Bdi (intrinsic remanant induction) of a magnet sample in an open circuit condition. The
Helmholtz coil arrangement is also fairly independent of the position of the sample within the
coils. Several parameters should be considered when selecting a Helmholtz coil for a
particular application.
4.7.1 Helmholtz Coil Diameter
A Helmholtz coil should be selected such that the largest dimension of the magnet sample is
less than one third the diameter of the Helmholtz coil. Normally the diameter of the Helmholtz
coil is a compromise between signal strength and the independence of the magnet sample
position.
4.7.2 Helmholtz Coil Resistance
As with the search coil, the resistance of the Helmholtz coil should normally be less than 10%
of the Model 2130 input impedance. Refer to section 4.6.2 “Coil Resistance” for more details.
4.7.3 Helmholtz Constant
In order to make an accurate measurement of the magnet sample Bdi, the Helmholtz coil
constant must be known, and entered into the Model 2130 coil parameter menu. Coils
purchased from Magnetic Instrumentation, Inc. include this constant.
Summary of Contents for 2130
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