Lake Shore Model 480 Fluxmeter User’s Manual
Magnetic Measurement Overview
2-9
2.2.6 Field Uniformity
Flux measurement is a true indication of lines of flux passing through a coil. Field uniformity does not
affect flux measurement, but other magnetic measurements such as flux density assume uniform flux
over the coil area. When measuring flux density in a non-uniform field, the fluxmeter reads the
average flux density.
There are some unique coil configurations that help eliminate the effect of field non-uniformity. The
length to outer diameter ratio of a coil can be optimized to measure flux density at the center of the
coil rather than the average flux density. For more information consult:
Zijlstra, H.
Experimental Methods in Magnetism
, Wiley, pg. 3, 1967.
Herzog & Tischler,
Measurement of Inhomogeneous Magnetic Fields
, Review of Scientific
Instruments, Vol. 24, pg. 1000, 1953.
2.2.7 Lead Pickup
Loops other than the sensor coil should be eliminated or minimized. Loops in lead wires see changing
flux just like a coil. Their voltage is an error added or subtracted from the coil voltage. Twisted leads
from the coil to the fluxmeter are recommended to reduce loop area and minimize error voltage.
2.2.8 Inductance, Capacitance, and Self Resonance
There are error sources that are only important when making AC or very fast peak DC
measurements.
Keep coil inductance (
L
coil
) small, or it acts similar to coil resistance and reduces sensitivity. The real
impedance of a coil due to inductance is
2
f L
coil
. That value should be small compared to the input
resistance of the integrator, or the signal is attenuated. The attenuation changes with frequency
because the impedance does. The equation for calculating inductance of an ideal long solenoid is:
where
0
= 4
10
-7
H/m
,
N
= turns,
A
= area in m
2
,
l
= length in m, and
L
coil
is in henries (H).
Equations for flat search and Helmholtz coils are more complicated because there is no simple
relationship between inductance and length, but the effects of area and number of turns remain
consistent.
There is capacitance between each turn of wire in a coil (
C
coil
). Impedance resulting from the
capacitance itself is most often negligible at frequencies below 50 kHz. However, the capacitance
reacts with the coil inductance to make the coil resonate. Operating anywhere near the coil resonant
frequency gives unpredictable results. The frequency of resonance is:
2.2.9 Lake Shore Coils and Probes
It may be desirable to purchase pre-fabricated sense coils optimized for Model 480 use. Lake Shore
offers search and Helmholtz coils. Dimensions and specifications appear in Chapter 7. They are
designed for every day use with well secured windings and strain relief at connection points.
Factory calibration ensures accurate measurements from the start without field calibrating the coil in a
magnet standard. They also ensure interchangeability of probes and fluxmeters for reproducible
measurements. Lake Shore calibrations use the most accurate standards available. Each coil comes
with calibration data that may include number of turns, area, and resistance.
Lake Shore sense coils are very easy to use. Calibrated coil parameters are usually pre-programmed
into non-volatile memory in the connector. Users need only plug in the connector, turn the power on,
and begin taking measurements.