December 1959 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Hall devices are used not just for magnetic field measurements,
but also for for current and power measurements and as function
generators, transducers, multipliers, and isolators. The Hall
effect comes into play when a stationary current-carrying
conductor is located in a magnetic field, where electrons within
the conductor are forced into a distribution that favors one
side of the conductor. The result is a difference of potential
across the conductor which is directly related to magnetic field
strength by a well-defined equation, permitting measurement
of the magnetic field. A gaussmeter works on that principle.
Since a Hall device's output is proportional to the product
of the control current and the magnetic field, and the magnetic
field determines the Hall voltage, power can be determined with
the devices as well.
The Hall Effect
By John R. Collins
Great progress is being made toward moving a rather ancient
principle - the Hall effect - out of the laboratory and into
practical use. Electronics exhibits, manufacturers' catalogues,
and advertisements display an increasing number of products
incorporating Hall devices now available for purchase. And the
end is not in sight - new applications appear continually in
the literature. Unquestionably there are still more Hall devices
on drawing boards than on production lines.
The phenomenon was recognized long ago as
a curiosity. New materials are rapidly moving it out of the
laboratory and into a growing number of practical devices.
While the Hall effect dates back to 1879, it remained a sort
of scientific curiosity for more than half a century. With the
advance of semiconductor technology, however, it has become
possible to make Hall units with the necessary sensitivity,
stability, and output.
The major emphasis thus far has been on instruments for measuring
magnetic fields or determining the magnetic properties of materials.
In addition, however, Hall devices are used for current and
power measurements and as function generators, transducers,
multipliers, and isolators.
The Hall effect refers to the influence a magnetic field
has on a stationary current-carrying conductor. Perhaps the
easiest way to visualize the effect is by thinking of the familiar
motor principle. A current-carrying conductor which is cut by
a magnetic field will be subjected to a thrust which will cause
it to move at right angles to both the direction of the current
and the direction of the magnetic field. This principle is the
basis of operation of d.c. motors and the D'Arsonval meter movement.
A natural question is, what happens if the conductor is restrained
so that it is not free to move? Edward H. Hall found the answer
through his experiments at Johns Hopkins University more than
80 years ago. Using a thin strip of gold leaf as a conductor,
Hall showed that a difference in potential ("Hall voltage output"
in the diagrams) will appear across opposite edges of the strip
under these conditions. This is illustrated in Fig. 2A.
Fig. 2. (A) Hall voltage develops in the
conductive strip. (B) Path of deflected electron. Direction
of flux is into page.
The explanation of the effect lies, of course, in the deflection
of electrons in the current stream to one side of the conductor
by the influence of the magnetic field, as shown in Fig. 2B.
The same sort of deflection may be observed in a magnetic-deflection
type cathode-ray tube. In some semiconductors, positive charges
("holes") are the majority carriers, and these are also deflected
by the magnetic field. The concentration of like charges at
one edge of the conductor produces a difference in potential
as would be expected. This is called the Hall voltage.
The magnitude of the Hall voltage is directly related to
the strength of the magnetic field. As electrons are deflected
to one side, the negative charge thus produced tends to oppose
the movement of more electrons to that edge. A condition of
equilibrium is reached when this repelling force exactly balances
the impelling force of the field's magnetic strength.
The relationship among the several factors which determine the
Hall voltage is expressed by the following formula:
Vh = Rh/d X I X B X sin θ. Vh
= Hall voltage in volts, Rh = Hall coefficient (described
below), I = current in amperes, B = magnetic flux density in
gauss, d = thickness of conductor in centimeters, and θ
= angle between I and B. In the usual case, where I and B are
at an angle of 90° with respect to each other, the sine
of θ is 1 and can be ignored.
Early experiments with the Hall effect were limited because
of the lack of suitable materials and although many different
metals were tried (including antimony, cobalt, sodium, and zinc),
the results were far from satisfactory. Resistance of the materials
was low, making it difficult to obtain a Hall voltage large
enough for practical purposes. When materials of higher resistance
were used, the efficiency declined to such an extent that the
device became virtually useless.
Experiments show that there are two factors which determine
the suitability of materials for Hall effect use. First, the
mobility of the charge carriers (either electrons or holes)
must be high. This is obviously important since carrier mobility
determines the response to an applied force and hence the sensitivity
of the device.
High mobility, however, tends to reduce the resistance of
the device so that impedance matching becomes a serious problem.
The best way to overcome this difficulty without lowering the
efficiency is to reduce the number of carriers by purifying
the material. These considerations are expressed in the following
formula for the Hall coefficient: Rh = μ/ς
where μ is the mobility, a measure of drift velocity of the
carriers, expressed in terms of drift per centimeter per second
for a potential gradient in volts per centimeter, and ς
is a measure of the concentration of the carriers per cubic
centimeter of the material.
The Hall coefficient thus serves as a figure of merit in
selecting materials for use in Hall devices. Indium anti-monide
(InSb) has the greatest carrier mobility and hence the highest
efficiency of any material thus far discovered. It is quite
temperature-sensitive, however, and this fact has limited its
use. Instead, many Hall devices now employ either indium arsenide
(InAs) or indium arsenide phosphide (InAsP). These materials
have a carrier mobility substantially greater than either germanium
or silicon and, while not as efficient as InSb, they are considerably
less temperature-dependent than indium antimonide.
A Hall generator is a solid-state multiplying device which
is designed to produce a Hall output voltage proportional to
the product of the control current and the magnetic field. It
is constructed by forming semiconductor material into a thin
wafer and attaching leads to each of the four sides. Two opposite
leads are for conducting the control current, and the other
two are the Hall output voltage leads. The wafer is attached
to a thin, insulating panel and the unit is encapsulated in
The Hall-voltage formula above shows that the output increases
as the thickness of the conducting wafer decreases. It is advantageous,
therefore, to make the wafer as thin as mechanical strength
will permit. In addition, the panel on which it is mounted should
be made thin so as to fit into the small air gaps found in practical
Intermetallic elements, such as InAs and InSb are very brittle,
making it difficult to machine them to very thin sizes. A solution
to this problem is to vacuum-deposit the material in a thin
film on a glass or ferrite substrate and to connect leads to
opposite sides before encapsulation.
The Helipot Division of Beckman Instruments, Inc. produces
units in this manner, depositing InSb in a film only 7 microns
(0.00028") thick on a substrate plate 0.012" thick. These units
are extremely sensitive because of their thinness and because
of the high carrier mobility of InSb. Some will give Hall output
voltages of 2 volts per ampere-kilogauss.
Deposited-film construction provides a large surface-to-volume
ratio which helps dissipate heat - an important factor since
InSb, as already mentioned, is extremely temperature-sensitive.
The thinness of the film places a limit on the amount of current
that can be handled, however, so it is necessary to strike a
balance among several variables in order to obtain a sensitive
Hall generator which will have reasonable dimensions and still
be capable of handling practical currents.
Fabrication of Hall generators is a delicate operation which
requires careful attention to construction details. One source
of error results from improper alignment of the Hall output
voltage leads on the edges of the wafer. Unless these are exactly
positioned at equipotential points, a voltage will exist between
the contact points whenever there is current flow, even with
zero magnetic field. The effect, known as the resistive null
voltage, can be compensated for by a resistive network, but
unless this is done the generator will give false readings and
the error will increase with the current.
The speed of response of a Hall generator is extremely fast,
ranging well into the megacycle region. By itself, however,
a Hall generator is an incomplete unit, since a magnetic circuit
is needed for its operation. The inherent slowness of magnetic
circuits usually limits the final speed which can be attained
in Hall circuits.
One of the most important applications of the Hall generator
is detecting and measuring magnetic fields. For this purpose,
it has the advantage over more conventional instruments in that
relative motion is not needed between the magnetic field and
the pickup element. It is only necessary that the control current
(which can be either a.c. or d.c.) be a known quantity. Hall-effect
gaussmeters are shown in Figs. 3 and 10.
Since it is often necessary to measure magnetic flux in narrow
air gaps, the Hall generator is usually mounted in a probe which
is connected through a cable to the gaussmeter instrument. Various
kinds of probes have been developed to meet the different conditions
of use. A transverse probe Fig. 1) is used to detect fields
directed toward the probe's flat side, while an axial probe
(Fig. 7), with a small Hall generator mounted flat at its end,
is used for fields directed towards the tip. Probes less than
0.02" thick are available for making measurements in very narrow
Fig. 1. Flat probe by Radio Frequency Labs
used to detect transverse fields. InAs element is embedded near
Fig. 3. Sensitive gaussmeter by Bell measures
Fig. 7. An axial gauss meter probe, with
element mounted flat at its end, for detecting fields directed
toward its tip.
Fig. 10. A portable gaussmeter by Instruments
Systems Corp., with its probe.
Through the use of a probe, an operator can determine the
area where flux density is greatest and, since the Hall output
is greatest where the generator element is perpendicular to
the magnetic field, it is easy to find the field direction.
Information of this kind is especially useful when checking
for flux leakage in the vicinity of transformers or relays.
For very accurate work, the gauss meter is zeroed while the
probe is in a special zero-gauss chamber which shields the sensitive
element from the earth's field (about 0.5 gauss) and any stray
magnetic fields. Also, standard magnets of known strength are
used to calibrate the gaussmeter.
A very sensitive gaussmeter with 12 full-scale ranges from
0.1 gauss to 30,000 gauss is shown in Fig. 3 and in block diagram
form in Fig. 9. The circuit is entirely transistorized. An oscillator
feeds a 1100-cps, 100-ma. control current through the sensing
element. In the presence of a magnetic field, the Hall output
voltage will be an 1100-cycle voltage which is amplitude-modulated
by the magnetic field being measured.
Fig. 9. How the amplified Bell gaussmeter
of Fig. 3 works.
A portion of the oscillator output is fed back into a summing
circuit where it is used to cancel out a major portion of the
signal caused by the magnetic field. High amplifier gain then
permits small changes in field strength to be studied in detail.
After amplification, the signal is demodulated to obtain a d.c.
or a low-frequency a.c. signal proportional to the magnetic
field. This signal deflects the panel meter and may also be
used to operate control circuitry or an oscillograph.
Gaussmeters of this kind are especially useful for studying
small changes in a magnetic field, hence their name "incremental"
gaussmeters. Since the signal and the detector are synchronized,
the circuit rejects out-of-phase signal components, a factor
which tends to reduce the effects of noise. Among other applications,
they are useful for evaluation of magnetic ink, tape, and magnetic
A different gaussmeter is shown in Fig. 8. It utilizes two
matched probes which are mounted parallel to each other at a
fixed distance apart. The system is supplied by a 3000-cycle
oscillator, and separate constant-current amplifiers furnish
the control current to each probe. The current through each
element is held constant to insure that its output will be exactly
proportional to the magnetic field at that point. The Hall voltage
output of one probe is inverted and added to the voltage output
of the other - which amounts to subtracting the two outputs.
The difference voltage is displayed on the meter, which can
be detached from the instrument itself for convenient viewing.
Fig. 8. Two matched probes are used in this
differential gauss meter by RFL. It can detect flaws in magnetic
Differential voltmeters of this kind are useful for measuring
the gradient of the magnetic field in connection with locating
and measuring flaws, anomalies, and residual magnetism within
ferromagnetic materials. Uniform fields, such as the earth's,
do not affect the readings regardless of the position of the
It is notable that many sensitive gaussmeters employ a.c.
for the control current and thus have an a.c. Hall output voltage.
The advantage is that a.c. is easier to maintain at a constant
level since it is not subject to the drift problem encountered
in d.c. Also, such a.c. voltages are much easier to amplify.
While many Hall-effect gaussmeters are employed for intermittent
measurements, they are also useful in continuous applications.
One of the most demanding applications is the monitoring and
controlling of the magnetic field of a mass spectrometer. The
field must be maintained at an extremely constant level in order
to make possible the delicate analysis carried on by a mass
spectrometer. Even when the voltage of the electromagnet power
supply is kept at a constant level, the current and the magnetic
field may vary because of resistance changes due to temperature
fluctuations. A Hall-effect gaussmeter is used to monitor the
field and to act as the sensing element in an automatic control
Hysteresis Curve Tracer
A special adaptation of the Hall effect is in the measurement
of the hysteresis of magnetic materials. The key to the way
a magnetic core will perform in a particular application is
its hysteresis curve, which shows how the flux density in the
core varies with the cycling of an a.c. magnetic field. A fast,
accurate means of plotting the hysteresis curve is a necessity
for development work and in manufacturing many kinds of magnetic
devices. Hall generators are especially useful for such analysis
because they measure the instantaneous field without any time
lag. The hysteresis loop can be displayed on an oscilloscope.
The basic elements of such a curve tracer are shown in Fig.
4. The magnetic core to be tested is placed in the center of
a coil through which is passed an a.c. current which is in phase
with the control current. A voltage drop is obtained by means
of a resistor inserted in the path of the winding and the voltage
thus obtained is applied to the horizontal plates of the oscilloscope.
The Hall device is placed in contact with the magnetic test
material so that the magnetic field will be perpendicular to
it. The Hall voltage thus derived is applied to the vertical
plates of the oscilloscope. The result will be a typical hysteresis
Fig. 4. Test assembly for tracing hysteresis
curve on scope.
A Hall generator, as mentioned before, is fundamentally a
multiplying device, producing an output that is proportional
to the product of the control current and the magnetic field.
Since power is determined by the product of current and voltage
(P = E x I), it is only necessary to have the control current
proportional to the circuit voltage and the magnetic field proportional
to the circuit current in order to obtain a Hall voltage proportional
to the circuit power.
A generalized wattmeter circuit employing a Hall generator
is shown in Fig. 5. A shunt, R, is provided so that only a portion
of the circuit current flows through the coil and a series dropping
resistor, Rs, limits the voltage applied to the Hall
generator. The selection of values for the two resistors depends,
of course, on the amount of power involved.
Fig. 5. Hall wattmeter renders power as product
of E and I.
In a practical wattmeter capable of measuring power in a.c.
circuits at frequencies from 50 to 500 cps, the magnetic field
is provided by a coil in the line circuit, and the control current
is obtained from a stepdown transformer connected across the
load terminals. The output is a double-frequency wave superimposed
upon d.c. The double-frequency component is proportional to
volt-amperes while the d.c. component is proportional to watts.
The Hall effect is extremely fast and if an oscilloscope
is used as the output device instead of a conventional meter,
it is possible to show instantaneous power. This is useful in
studying transients which occur as fault currents when a circuit
has been broken or interrupted.
A hook-on ammeter is an instrument which can be used to measure
current in a conductor when it is in close proximity to or hooked
onto the conductor. The fact that the circuit does not have
to be opened to allow the meter to be inserted in the current
path is a great advantage, especially for measuring high bus-bar
currents, and a convenience that saves both time and effort
with a current of any magnitude.
By using a split C-yoke with a Hall generator (Fig. 6) in
the air gap, a hook-on ammeter can be made which overcomes many
of the difficulties in some other types of instruments. The
control current, which can be either a.c. or d.c., is maintained
at a constant level. The magnetic field which surrounds the
bus bar supplies the field for the Hall generator. Since this
field is proportional to the current in the bus bar, the Hall
output voltage will also be proportional to the bus-bar current.
Fig. 6. Hall element and split C-yoke make
Many other ways of utilizing Hall generators have been devised.
Some are already in limited use, many are still theoretical.
A frequency doubler, for example, can be constructed using a
Hall generator. If the same a.c. source is used for both the
control current and the magnetic field, the Hall-output voltage
will have an a.c. component with a frequency twice the frequency
of the input. Similarly, the Hall generator can be used as a
device for squaring functions. If the magnetic field and the
control current are both driven by the same signal source, the
output will be proportional to the square of the input.
The Hall generator can also be used as a function generator
in an analog computer. In the discussion above, it was pointed
out that the Hall output voltage is proportional to the product
of the control current, the magnetic field, and the sine of
the angle between I and B. Accordingly, if the Hall generator
is rotated in the magnetic field, it will produce a sine wave
(or a cosine wave, depending on the point of reference).
Conventional function generators produce outputs whose amplitudes
depend on the rate at which magnetic lines of force are cut
by a conductor. The output of the Hall generator is independent
of the speed of rotation, however, and it is therefore especially
well adapted for very-low-frequency operation.
When very small d.c. signals must be measured, it is often
convenient to convert them first to a.c., so that they can be
readily amplified and to eliminate drift problems. A Hall generator
is useful for this operation, which is known as "chopping."
An a.c. magnetic field is employed, and the low-level d.c. voltage
is impressed across the Hall control terminals. The output of
the generator is then an a.c. signal that is equal to the product
of two quantities: the magnetic field and the control current
resulting from the d.c. voltage.
Another application in which Hall generators show promise
is as isolators in microwave circuits. An isolator is a four-terminal,
unidirectional, transmission device. Electron tubes and transistors
used as amplifiers are examples of isolators, since they operate
in one direction only.
A tunnel diode, on the other hand, is a two-terminal, bidirectional
device. The fact that tunnel diodes have common input and output
terminals makes it difficult to build multi-stage amplifiers
in which they are employed. Effort is currently being devoted
to using Hall devices in conjunction with tunnel diodes. If
it is successful, the usefulness of both devices will be broadened
by another configuration providing the necessary isolation of
the input and output circuits.
Posted February 18, 2015