6.5 Measuring Electric and Magnetic Fields
189
Electrometrics
Electrometers are devices used to measure electric potentials, electric fields, and/or
electric currents.
A valve electrometer uses a vacuum tube with a grid and a plate. Currents as low
as one femtoamp (10 −15 A) can be measured.
Solid-state electrometers that used field-effect transistors (FETs) have an input
impedance of about 10 14 , again making the required trickle current during
measurement extremely low, and give fractions of a picoCoulomb sensitivity. These
kinds of sensitivities are useful for detecting individual nerve actions.
Magnetometrics
The first detection of a magnetic field came from the observation of pieces of
lodestone acting on each other. Soon after, when a piece of lodestone was allowed
to rotate in a horizontal plane, the first compass was constructed in China at about
300 BC.
These days, magnetic field detectors, referred to as ‘Gaussmeters’, often use a
moving coil of wire or a solid-state Hall effect device. If a coil of wire is rotated in
a static magnetic field, an alternating electric field is induced in the wire, and will
generate a current which can be measured. The Hall effect occurs when a conductor
carrying a current is placed in a magnetic field. The field pushes the moving charges
to the side (from the Lorentz force qv × B). The potential from side to side of the
conductor will be of magnitude V = LvB, where L is the width of the conductor.
Thus, B will be proportional to the induced voltage.
Hall effect magnetometers can be made small enough to be incorporated onto
the circuit boards of mobile smart-phones, thus giving them compass direction
capabilities.
Alternative magnetometric devices include ones that use the fact that external
magnetic fields can affect the energy states of orbiting and spinning charged
particles in bound states of atoms and molecules, since such moving and spinning
charges will feel and be affected by magnetic forces. The effects on energy states
due to typical magnetic fields are small, but easily detectable using resonance
measurements with an external electromagnetic wave stimulator. These devices
can give magnetic field measurements accurate to 1 part per million. A common
type of magnetometer uses proton-spin-precession. Water or other hydrogen bearing
material is placed within a coil of electric wire capable of producing a uniform 5–
10 mT magnetic field. When the coil is energized, protons within tend to ‘align’ their
spins in the direction of the field. When the coil current is switched off, the protons
spins tend to flip to the direction of the external field, releasing photons as a radio
wave which can be detected. The frequency of this wave depends on the strength
of the external magnetic field. Commercial proton-precession magnetometers are
sensitive to about a tenth of a nanotesla.
189
Electrometrics
Electrometers are devices used to measure electric potentials, electric fields, and/or
electric currents.
A valve electrometer uses a vacuum tube with a grid and a plate. Currents as low
as one femtoamp (10 −15 A) can be measured.
Solid-state electrometers that used field-effect transistors (FETs) have an input
impedance of about 10 14 , again making the required trickle current during
measurement extremely low, and give fractions of a picoCoulomb sensitivity. These
kinds of sensitivities are useful for detecting individual nerve actions.
Magnetometrics
The first detection of a magnetic field came from the observation of pieces of
lodestone acting on each other. Soon after, when a piece of lodestone was allowed
to rotate in a horizontal plane, the first compass was constructed in China at about
300 BC.
These days, magnetic field detectors, referred to as ‘Gaussmeters’, often use a
moving coil of wire or a solid-state Hall effect device. If a coil of wire is rotated in
a static magnetic field, an alternating electric field is induced in the wire, and will
generate a current which can be measured. The Hall effect occurs when a conductor
carrying a current is placed in a magnetic field. The field pushes the moving charges
to the side (from the Lorentz force qv × B). The potential from side to side of the
conductor will be of magnitude V = LvB, where L is the width of the conductor.
Thus, B will be proportional to the induced voltage.
Hall effect magnetometers can be made small enough to be incorporated onto
the circuit boards of mobile smart-phones, thus giving them compass direction
capabilities.
Alternative magnetometric devices include ones that use the fact that external
magnetic fields can affect the energy states of orbiting and spinning charged
particles in bound states of atoms and molecules, since such moving and spinning
charges will feel and be affected by magnetic forces. The effects on energy states
due to typical magnetic fields are small, but easily detectable using resonance
measurements with an external electromagnetic wave stimulator. These devices
can give magnetic field measurements accurate to 1 part per million. A common
type of magnetometer uses proton-spin-precession. Water or other hydrogen bearing
material is placed within a coil of electric wire capable of producing a uniform 5–
10 mT magnetic field. When the coil is energized, protons within tend to ‘align’ their
spins in the direction of the field. When the coil current is switched off, the protons
spins tend to flip to the direction of the external field, releasing photons as a radio
wave which can be detected. The frequency of this wave depends on the strength
of the external magnetic field. Commercial proton-precession magnetometers are
sensitive to about a tenth of a nanotesla.