There are many applications in the design of rotating electric machines (alternators,
dc generators and motors, brushless dc motors, stepping motors, induction motors, etc.)
IX
Vo
B
Hall sensor
Air gap
Magnetic core
I
Current-carrying
conductor
FIGURE 8.43
A Hall sensor used to measure the ac
current in a conductor. The Hall output
voltage is proportional to the product of BY
IH, hence I.
550 Introduction to Instrumentation and Measurement
where it is useful to be able to measure the magnetic field strength in air gaps in order to
verify designs. There are also many applications in physics and chemistry, where it is
necessary to measure magnetic fields (e.g., in mass spectrometers, magnetic resonance
imaging systems and in particle accelerators). Recently, power line frequency magnetic
fields have been implicated as possible contributing causes in health problems including
birth defects, leukemia and cancer [EPRI Journal Jan/Feb 1990]. Thus, measurement of the
spatial distribution of 60 Hz B fields in areas containing people, experimental animals or
cell cultures is important for the clarification of the significance of 60 Hz B fields in health
problems. The magnetic field vector, B, is also called the magnetic induction or the magnetic
flux density. Its units are newtons per coulomb divided by meters per second, or
equivalently, volt seconds per square meter, or webers per square meter. The more
common units for B are gauss (1 weber/m2¼104 gauss), or teslas (104 gauss¼1 T).
Magnetic flux is defined as the integral of B over an area perpendicular to B, or: