Garshelis Effect

The Garshelis effect is a small, specialised piece of magnetism. Its usual one-line definition says it makes springs made of magnetostrictive material change their magnetization when the spring is compressed. The effect is named after Ivan J. Garshelis, an American engineer and researcher who spent decades studying how mechanical stress and magnetization interact in iron alloys. Most of his work was aimed at measuring torque on rotating shafts without touching them.

On its own the definition is thin, and the encyclopedia entry behind it is a short stub. It makes sense only inside a larger family of "magnetomechanical" effects that physicists have catalogued since the 1840s. In these effects, magnetizing a piece of iron changes its shape, and squeezing, stretching or twisting it changes its magnetization. The Garshelis effect is one member of that family. It ties the compression of a coiled spring to a change in the spring's magnetization, and when the magnetization comes from a direct current flowing through the wire, it is the inverse of the Wiedemann effect.

This article explains the effect in that wider context. It covers the physics of magnetostriction, why a compressed coil spring is really a twisted wire, how the Wiedemann effect and its inverse work, and how Garshelis's research led to a widely used non-contact torque sensor. It also covers what the effect can and cannot do, common misconceptions, and some frequently asked questions.

The Short Definition and Where It Comes From

The Wikipedia article on the Garshelis effect describes it as a link between magnetization and torsional stress in magnetostrictive springs. Its core sentence says the effect "causes springs made of magnetostrictive material to have their magnetization changed due to the compression of the spring." The article adds that when the magnetization is due to direct current, the effect is the inverse of the Wiedemann effect.

The entry rests on two sources:

• **Harry E. Burke, Handbook of Magnetic Phenomena (1986).** This engineering reference has a chapter on magnetostriction that groups and names the various magnetomechanical effects.
• **I. J. Garshelis, "A Study of the Inverse Wiedemann Effect on Circular Remanence," IEEE Transactions on Magnetics, vol. MAG-10, no. 2, pp. 344–358 (1974).** Later patents on magnetoelastic torque sensors cite this paper by that title and page range.

The title of the 1974 paper is a strong clue. Its subject is the "inverse Wiedemann effect" acting on "circular remanence," meaning the magnetization left circling a wire or rod after a current through it has been switched off. That is the physics behind the named effect. A coiled spring is just a convenient way to apply the right kind of stress to a magnetized wire.

Many reference works never use the phrase "Garshelis effect." It appears mainly in handbook-style lists of magnetomechanical effects. Specialists more often describe the same physics as the inverse Wiedemann effect, the Matteucci effect, or more generally the magnetoelastic or inverse magnetostrictive effect. Garshelis's own patents use "magnetoelastic" for how elastic torsional strain affects magnetization. A Garshelis-related patent notes that other inventors in the field prefer "magnetostrictive" for the same thing. Readers who search the technical literature will find far more under those terms than under the eponym.

Background: Magnetostriction

What magnetostriction is

Magnetostriction is the property of magnetic materials that makes them change shape or dimensions when they are magnetized. As the applied magnetic field increases, the strain grows until it reaches a saturation value, written λ (lambda). The magnetostrictive coefficient λ is the fractional change in length as the magnetization rises from zero to saturation. It can be positive, meaning the material lengthens along the field, or negative, meaning it shortens.

The changes are tiny. A ferromagnetic rod's length changes by a few parts per million when it is magnetized. Volume changes on magnetization are also small, on the order of one part in a million. These small numbers explain why magnetostriction took scientific attention and fine instruments to discover. They also explain why the humming of transformers, which is caused by magnetostriction, is a nuisance rather than a hazard.

Who discovered it

James Prescott Joule first identified the effect in 1842 while observing a sample of iron. Joule is better known for his work on heat and energy, but his magnetostriction experiments came first. The basic change in length is still called "Joule magnetostriction" in his honour.

Why it happens: domains and anisotropy

Ferromagnetic materials such as iron, nickel and cobalt are divided into domains. A domain is a small region in which the magnetization points in one uniform direction. In an unmagnetized piece of iron the domains point in many directions and their effects cancel. Applying a field moves the walls between domains and rotates the magnetization within them. Both processes change the material's dimensions.

The deeper cause is magnetocrystalline anisotropy. In a crystal, magnetization is easier along some directions, called easy axes, than along others. In iron, for example, the cube-edge directions of the crystal, written [100], are the easy directions. When a field is applied at an angle to an easy axis, the material rearranges so that its magnetization lies closer to the field, which lowers its free energy. Because the crystal is slightly distorted in a way that depends on where its magnetization points, that rearrangement produces strain.

How big the effect is in common materials

Magnetostriction varies a great deal from material to material. The table gathers representative values from the Wikipedia article on magnetostriction and from published thin-film measurements. The values are saturation strains at room temperature. One microstrain is one part per million.

The nickel figure comes from a thin-film study that reported about −38 parts per million and said this agreed with published bulk values. Another paper fitted nickel-cobalt alloys with the expression λs = (−38 + 0.84C) × 10⁻⁶, where C is the weight per cent of cobalt, which also gives about −38 ppm for pure nickel. For polycrystalline materials, the average saturation magnetostriction can be estimated from the two single-crystal constants with λs = (1/5)(2λ₁₀₀ + 3λ₁₁₁).

The Villari reversal in iron

Iron behaves unusually. In fields of about 40 kA/m, its magnetostriction changes sign from positive to negative. This is called the Villari reversal. It means the direction of iron's magnetoelastic response depends on how strongly the iron is magnetized. Partly for this reason, practical stress sensors use carefully chosen alloys rather than pure iron.

Hysteresis

Magnetostriction also shows hysteresis. Plotted against the magnetizing field, the strain traces a loop, much as magnetic flux density does. Because of its shape, the loop is sometimes called a "dragonfly loop." Engineers model it with tools such as the Jiles–Atherton model. For a sensor designer, hysteresis means the output can depend on the material's history as well as its present state. Much of Garshelis's later engineering work aimed at designs that keep this problem small.

The Family of Magnetomechanical Effects

Magnetostriction is the best known of several related effects. Each describes a different way that mechanics and magnetism are coupled. The Garshelis effect is easiest to understand once the whole family is laid out.

Two pairs in the table matter most here. The first pair is magnetostriction and its inverse, the Villari effect. The second pair is the Wiedemann effect and its inverse. Sources disagree about how to name that inverse. Wikipedia's article on magnetomechanical effects calls the Matteucci effect the inverse of the Wiedemann effect. Its magnetostriction article describes the Matteucci effect as torque creating helical anisotropy of the susceptibility. Those are two descriptions of closely related phenomena, and the naming has never been perfectly tidy. The Garshelis effect fits in this same corner of the table. It is a particular case of the inverse Wiedemann effect, produced by a particular mechanical arrangement.

The Villari Effect: How Stress Changes Magnetization

Definition

The inverse magnetostrictive effect is also called the magnetoelastic effect or the Villari effect, after Emilio Villari. It is the change in a material's magnetic susceptibility when it is subjected to mechanical stress. Magnetostriction says that magnetizing a material changes its shape. The Villari effect says that stressing a material changes how easily it magnetizes and how much magnetization it holds.

The energy picture

The cleanest way to see why stress affects magnetization is to write down an energy. For a single stress σ acting on a single magnetic domain, the magnetoelastic energy density is:

E_σ = (3/2) λs σ sin²θ

Here λs is the saturation magnetostriction and θ is the angle between the magnetization and the direction of the stress. By convention tensile stress is positive and compressive stress is negative.

The magnetization settles at whatever angle makes the total energy lowest. The formula leads to some simple rules:

• If λs σ is positive (for example, positive magnetostriction under tension), the energy is lowest at θ = 0. The magnetization prefers to lie along the stress, and the flux density along that axis rises.
• If λs σ is negative (for example, positive magnetostriction under compression, or negative magnetostriction under tension), the energy is lowest at θ = 90°. The magnetization prefers to lie across the stress, and the flux density along the stress falls.

For iron under tension, with λs and σ both positive, the energy is minimised at θ = 0. Tension therefore lines the domains up with the stress and increases the magnetization along it.

Worked example: the size of the magnetoelastic energy (hypothetical)

Take a hypothetical alloy with λs = 20 × 10⁻⁶ under a tensile stress of 100 MPa (1 × 10⁸ Pa). The coefficient in front of sin²θ is:

(3/2) × (20 × 10⁻⁶) × (1 × 10⁸) = 1.5 × 2,000 = 3,000 J/m³

So moving the magnetization from along the stress (θ = 0) to across it (θ = 90°) costs about 3,000 joules per cubic metre. The point of the example is the comparison. Soft magnetic materials can have built-in anisotropy energies of a similar order, so ordinary engineering stresses can noticeably steer the magnetization. That is why the Villari effect is large enough to build sensors around.

Measuring it

The Wikipedia article on the inverse magnetostrictive effect lists three requirements for a proper measurement. The sample needs a closed magnetic circuit, a uniform stress of known value and direction, and windings for measuring the hysteresis loop under stress. It describes four arrangements:

• Tension on a ribbon strip, which gives an open magnetic circuit.
• Tension or compression on a frame-shaped sample, which works only for bulk materials.
• Sideways compression on a ring core, which gives a non-uniform stress.
• Axial tension or compression on a ring sample, with the stress perpendicular to the field.

Each method trades simplicity against how cleanly it controls the stress and the magnetic path. A coiled spring offers yet another way to stress a wire, and, as the next section shows, a surprisingly uniform one.

Where it turns up

The Villari effect is used in force sensors, including sensors for civil engineering, for monitoring large locomotive diesel engines and ball valves, and for biomedical monitoring. It also appears as an unwanted side effect, for example in fluxgate magnetometer cores and in electric-motor stators that are fitted into housings with an interference fit, where the press-fit stress changes the core's magnetic properties.

Why Compressing a Spring Twists the Wire

The key mechanical fact behind the Garshelis effect is that the wire in a coiled spring is twisted, not compressed, when the spring is pushed together.

Coil-spring mechanics

Picture a helical compression spring of mean coil diameter D, made of round wire of diameter d, carrying an axial force F. Cut the spring at any point along the wire and look at what the wire must carry. The force F acts along the spring's axis, but the wire's centre is a distance D/2 from that axis. The wire section must therefore resist a twisting moment of about F × D/2 about its own centreline. It also carries a direct shear force F, which is a much smaller effect in a typical spring.

So the dominant stress in the wire is torsional shear. Standard mechanical-design texts give the nominal maximum shear stress in the wire, ignoring curvature corrections, as:

τ = 8FD / (πd³)

Designers usually multiply this by a correction factor to account for the wire's curvature and the direct shear, but the basic point holds. A compression spring is a torsion bar wound into a helix.

Worked example: spring wire stress (hypothetical)

Take a hypothetical spring with mean coil diameter D = 20 mm, wire diameter d = 2 mm, and axial load F = 50 N.

• Numerator: 8 × 50 N × 0.020 m = 8 N·m
• Denominator: π × (0.002 m)³ = π × 8 × 10⁻⁹ m³ ≈ 2.513 × 10⁻⁸ m³
• Shear stress: 8 / (2.513 × 10⁻⁸) ≈ 3.18 × 10⁸ Pa, or about 318 MPa

A modest 50-newton push, about the weight of a 5-kilogram mass, produces a shear stress in the hundreds of megapascals in a thin wire. That is far larger than the stress the same force would produce if it simply squeezed a solid rod of the same wire. The coil geometry acts as a mechanical amplifier that turns a small axial load into a large, fairly uniform torsional stress along the whole length of the wire.

Torsion means tension and compression at 45 degrees

Pure torsional shear can be described in another way. In a twisted wire, the shear stress τ is equivalent to a tensile stress of +τ along one helical direction at 45° to the wire's axis and a compressive stress of −τ along the perpendicular helix. This is standard stress analysis, and it is what links torsion to the Villari energy formula. For a material with positive magnetostriction, the magnetization prefers to swing toward the tensile helix and away from the compressive one. For negative magnetostriction, the preference reverses.

The Wiedemann Effect and Its Inverse

The Wiedemann effect

Gustav Wiedemann discovered the effect in 1858. When a ferromagnetic rod carries an electric current while sitting in a longitudinal magnetic field, the rod twists. The current makes a circular magnetic field around and inside the rod. Added to the longitudinal field, it gives a helical total field. Through magnetostriction, the helically magnetized material deforms into a twist. If either the current or the field alternates, the rod undergoes torsional oscillation.

In a linear approximation, the twist angle per unit length α depends on the current density j, a magnetoelastic parameter h₁₅ and the shear modulus G, through α = j h₁₅ / (2G). The magnetoelastic parameter grows with the strength of the longitudinal field. In this approximation the rod's cross-sectional shape does not matter.

The inverse: twisting a magnetized wire

Run the Wiedemann effect backwards. Start with a wire that is magnetized circularly, either by a current flowing through it or by the remanent circular magnetization a current leaves behind. Then twist the wire. The torsion creates the helical tension and compression described above, and through the Villari energy those stresses tilt the magnetization off the purely circular direction into a helix. A helix has an axial component, so the twisted wire develops axial magnetization it did not have before. If the twist is changing, that changing axial magnetization induces a voltage in a pickup coil, and a field appears at the wire's ends.

This is the inverse Wiedemann effect. Garshelis's 1974 paper studied exactly this situation, with circular remanence as the starting state.

Where the spring comes in

Now combine the two ideas. A coil spring is a twisted wire, and its twist is set by how hard the spring is compressed. Suppose the spring is made of magnetostrictive wire and carries circular magnetization, either from a direct current along the wire or from remanence. Compressing the spring twists the wire, the twist tilts the magnetization into a helix, and the axial component of that helix changes. Because the wire follows the coils of the spring, its local axial direction winds around the spring's axis. The net result is a change in the spring's magnetization that tracks the applied compression.

That is the Garshelis effect as the encyclopedia describes it. It is a geometric repackaging of the inverse Wiedemann effect. The spring turns an axial force into a fairly uniform torsion along a long length of wire, and the wire's circular magnetization turns that torsion into a measurable magnetic signal.

Step by step

Ivan Garshelis and the Magnetoelastic Torque Sensor

From effect to instrument

For most readers, the importance of Garshelis's work lies less in springs than in what the same physics made possible: measuring the torque on a rotating shaft without any contact.

Patent records list Ivan J. Garshelis as an inventor associated with Magnetoelastic Devices Inc. of Pittsfield, Massachusetts, and later with companies named Magna-Lastic Devices and MagCanica. His patents cite several stages of his research. They include the 1974 paper on the inverse Wiedemann effect, a 1975 presentation titled "The Wiedemann Effects and Applications," and a 1979 paper in the Journal of Applied Physics titled "Conditions for Stress Induced Bulk Moments." Public sources give little biographical detail beyond that patent and publication trail, so this article does not attempt one.

The problem being solved

Torque is the twisting effort a shaft transmits, and measuring it matters wherever power is transmitted: engines, gearboxes, pumps and steering columns. US Patent 5,465,627, granted to Garshelis on 14 November 1995 with a priority date of 29 July 1991, reviews the older approaches and their problems:

• Strain gauges bonded to the shaft measure strain through changes in electrical resistance. The patent describes them as unstable, of limited reliability because they need contact with a rotating shaft, and too expensive for competitive vehicle steering systems.
• Earlier non-contact magnetostrictive sensors, including Garshelis's own US Patent 4,896,544, used two magnetized bands with excitation and sensing coils. The coils took up considerable space and the sensors were too costly for price-sensitive uses.
• Permeability-based sensing in general had a deeper flaw. The patent argues that the measured flux density depends strongly on drive current, temperature, air gaps and leakage flux, and only weakly on torque. Differential designs with two sensors helped, but at the cost of size and complexity.

The motivating application was electric power steering, where an electric motor assists the driver in proportion to the torque the driver applies to the steering shaft. That needs a cheap, reliable, continuous torque signal in a harsh environment.

The circularly magnetized ring

Garshelis's solution, described in the 1992 IEEE Transactions on Magnetics paper "A torque transducer utilizing a circularly polarized ring" (vol. 28, no. 5, pp. 2202–2204), uses just two parts:

1. A ring of magnetostrictive material, rigidly attached to the shaft, which is magnetized around its circumference.
2. A Hall-effect device or similar magnetic field sensor mounted near the ring but not touching it.

With no torque on the shaft, the ring's magnetization runs in closed circles. A circle has no beginning or end, so no field leaks out and the sensor reads zero. When torque is applied, the stresses in the ring change its effective anisotropy from circular to helical. The magnetization tilts into a helix with an axial component. Because the axial magnetization stops abruptly at the ends of the ring, it creates a magnetic field in the space around the ring, much like the field of a short tubular bar magnet. The field's polarity shows the direction of the torque and its strength shows the magnitude. The paper reports that experimental units behaved as expected and showed a small, notably negative, hysteresis.

The physics is the same as in the Garshelis effect, applied to a different object. A shaft under torque has a twisted, circularly magnetized element. A compressed spring has a twisted, circularly magnetized wire. In both, torsion turns circular magnetization into helical magnetization with a measurable axial component.

Design details from the patent

The patent gives some practical detail:

• Material. The preferred ring material was 18 per cent nickel maraging steel (grade T-250). Other candidates included other maraging steels, Permendur-type alloys, certain stainless steels, and amorphous or nanocrystalline materials. The material has to be both ferromagnetic and magnetostrictive.
• Example geometry. One example was a ring about half an inch long, force-fitted onto a half-inch shaft, with a wall 0.030 to 0.050 inches thick.
• Built-in anisotropy. To make the circumferential direction the ring's preferred direction, the ring can be pressed, shrunk or interference-fitted onto the shaft. That creates hoop tension, which combines with positive magnetostriction. Cold rolling, knurling or heat treatment can also create the anisotropy.
• Shaft isolation. The shaft should have low magnetic permeability, or be isolated by a spacer or sleeve, so that it does not short-circuit the ring's flux.
• Sensitivity versus range. A large ratio of magnetostriction to anisotropy (λs/Ku) makes the magnetization easier to tilt and the sensor more sensitive. A smaller ratio gives a wider measuring range before the response saturates.
• No excitation needed. The output is zero at zero torque and does not depend on coil drive or alternating signals, which removes a major source of drift in older designs.

A 1992 SAE paper by Garshelis, Whitney and May (SAE 920707) described applying this transducer to electric power steering, calling it "a new magnetoelastic type of transducer of unusually small and simple construction." Later patents assigned to Magnetoelastic Devices, including US 5,351,555 and US 5,520,059, extended the design. A 2012 conference paper Garshelis co-authored described a magnetoelastic torquemeter system for monitoring and controlling the turboshaft engines of LCAC hovercraft, which shows the technology reaching into heavy marine propulsion.

Worked example: shaft stress in a sensing application (hypothetical)

The shear stress at the surface of a solid round shaft of diameter d under torque T is τ = 16T / (πd³). Take a hypothetical steering-type shaft with d = 20 mm and T = 10 N·m.

• Numerator: 16 × 10 = 160 N·m
• Denominator: π × (0.020 m)³ = π × 8 × 10⁻⁶ m³ ≈ 2.513 × 10⁻⁵ m³
• Shear stress: 160 / (2.513 × 10⁻⁵) ≈ 6.37 × 10⁶ Pa, or about 6.4 MPa

That stress is small compared with the spring example, and it is also small compared with a steel shaft's strength. A torque sensor therefore needs a material in which stresses of a few megapascals produce a clear magnetic response. This is why the choice of material and the built-in anisotropy matter so much. A ring pressed onto a larger shaft would see even lower stress, because surface stress falls with the cube of diameter.

Related Applications of Magnetomechanical Coupling

The Garshelis effect sits among several practical technologies that exploit the same coupling.

The magnetostrictive position sensor is the closest relative of the spring arrangement, because it also uses a wire. A small magnet marks the position along the wire. A short current pulse in the wire, combined with the magnet's field, launches a torsional pulse by the Wiedemann effect. A pickup at the end of the wire times its arrival, and a damper at the far end absorbs reflections. The Garshelis effect runs the same coupling in the opposite direction, from torsion to magnetization.

A Timeline of the Ideas

The physics behind the Garshelis effect built up over more than 150 years. The timeline below uses only dates given in the sources for this article.

The pattern is typical of applied physics. Nineteenth-century experimenters found and named the effects. They were then mostly of academic interest until twentieth-century materials, electronics and solid-state field sensors such as Hall-effect devices made it practical to use them. A Hall sensor small and cheap enough to sit beside a steering shaft did not exist in Wiedemann's time. Garshelis's work came when the old physics and the new electronics could finally be combined.

Limits and Practical Difficulties

The Garshelis effect and its relatives are real and well understood, but several practical limits shape how they are used.

Hysteresis and history

Magnetization depends on history as well as on the present stress. A spring or ring that has been heavily loaded, or exposed to a strong stray field, may not return exactly to its original state. Garshelis's ring design kept hysteresis small, and the 1992 paper reports it as small and negative. Even so, hysteresis is a central concern in any magnetoelastic sensor.

Temperature

Saturation magnetization, magnetostriction and anisotropy all change with temperature. A sensor calibrated at room temperature may read differently in a hot engine bay. Designers compensate with matched pairs of sensing regions, temperature measurement, or materials chosen for stable properties.

Stray magnetic fields

A sensor that reads a weak external field can be fooled by other fields, including the Earth's field and fields from nearby motors and cables. The patents describe soft-magnetic yokes that collect flux and designs that cancel uniform background fields.

Material stability

Remanent magnetization can decay over time, or be reset by shock or strong fields. The ring design relies on built-in anisotropy to keep the circular magnetization stable. A spring that relies on remanence faces the same issue. A spring that uses a continuous direct current avoids decay but needs a power supply and wiring.

Small signals from small strains

Magnetostriction strains are measured in parts per million, and practical stresses in torque applications can be only a few megapascals. The signal is real but small, and good electronics and careful design are needed to turn it into a reliable measurement.

Sign changes and nonlinearity

Iron's magnetostriction changes sign at about 40 kA/m, and many materials respond nonlinearly at high stress. A sensor must be designed to stay within the range where its response is predictable.

Common Misconceptions

"The Garshelis effect is a new kind of physics." It is not. It is a particular arrangement of the inverse Wiedemann effect, which itself follows from magnetostriction and its inverse. The physics was understood in the nineteenth century. Garshelis's contribution was careful study and practical engineering of it.

"Compressing the spring compresses the wire." For a helical compression spring, the wire is mainly in torsion. That is why the effect is torsional rather than a simple compressive Villari effect.

"Any steel spring will show a useful effect." All ferromagnetic steels are somewhat magnetostrictive, but the size and sign of the response vary widely with composition, heat treatment and magnetic state. Useful sensors depend on chosen materials and controlled anisotropy.

"Magnetostriction is large enough to see." Not in ordinary materials. Strains of a few parts per million need sensitive instruments. Giant-magnetostriction alloys such as Terfenol-D reach about 2,000 parts per million, which is still only 0.2 per cent.

"Magnetic torque sensors measure the shaft's magnetism directly." In older permeability designs the measured quantity depended heavily on things other than torque. The point of the circularly magnetized ring was to produce a signal that is zero without torque and depends mainly on torque.

"The Garshelis effect has health or energy applications." Nothing in the scientific record supports claims of that kind. It is a sensing and measurement phenomenon. Any product that cites it as the basis of a health or "free energy" device should be treated with great scepticism.

Frequently Asked Questions

What is the Garshelis effect in one sentence?

It is the change in magnetization of a coil spring made of magnetostrictive material when the spring is compressed. When the magnetization is driven by a direct current through the wire, it is the inverse of the Wiedemann effect.

Who was it named after?

It was named after Ivan J. Garshelis, who studied the inverse Wiedemann effect on circularly remanent wires in a 1974 IEEE Transactions on Magnetics paper. He later developed magnetoelastic torque sensors at Magnetoelastic Devices Inc. in Pittsfield, Massachusetts.

How is it different from the Villari effect?

The Villari effect is the general rule that stress changes magnetization. The Garshelis effect is a specific case. The stress is torsion, created by compressing a coil spring, and the starting magnetization is circular around the wire.

How is it different from the Wiedemann effect?

The Wiedemann effect runs from magnetism to mechanics. Combining a circular and a longitudinal field makes a rod twist. The Garshelis effect runs the other way. Twisting the wire, by compressing the spring, changes its magnetization.

Why use a spring at all?

A coil spring turns a small axial force into a large and fairly uniform torsional stress along a long length of wire. That makes the magnetic response bigger and easier to measure than squeezing a short solid rod would.

Is the effect used in commercial products?

The torque-sensing technology that grew from the same physics, the circularly magnetized ring, was developed for electric power steering and other shaft-torque uses, and it is described in many patents. The spring-specific form appears mainly in handbooks of magnetic phenomena rather than as a widely marketed device.

What materials work best?

Ferromagnetic materials with substantial magnetostriction and controllable anisotropy. Garshelis's torque patent preferred 18 per cent nickel maraging steel and listed other maraging steels, Permendur-type alloys, some stainless steels, and amorphous and nanocrystalline alloys.

Does temperature affect it?

Yes. The magnetic properties involved all change with temperature, so practical sensors need compensation.

Can I demonstrate it at home?

In principle, a spring of magnetic steel wire, a direct current and a sensitive pickup coil with an amplifier could show a voltage when the spring is squeezed. In practice the signals are small and easily swamped by other effects. Anyone trying this should take ordinary precautions with electrical current and heating of the wire.

Summary

The Garshelis effect is the change in magnetization of a magnetostrictive coil spring when it is compressed. The key to understanding it is the mechanics of springs. Compressing a coil spring twists its wire, and twisting a circularly magnetized wire tilts its magnetization into a helix with a measurable axial component. That is the inverse of the Wiedemann effect, which Gustav Wiedemann reported in 1858, and it rests on the magnetostriction James Joule first observed in 1842.

Ivan Garshelis studied this inverse effect on circular remanence in 1974. He went on to develop a non-contact torque sensor in which a circularly magnetized ring on a shaft produces an external field proportional to the torque, read by a Hall-effect sensor. That design, aimed at electric power steering, is the most important practical legacy of the physics behind his name. The effect is a good example of how a century-old laboratory curiosity, with strains of a few parts per million, can become a practical instrument once someone understands it well enough to engineer around its limits.

Sources

• Garshelis effect (Wikipedia)
• Magnetostriction (Wikipedia)
• Magnetomechanical effect (Wikipedia)
• Inverse magnetostrictive effect (Wikipedia)
• Wiedemann effect (Wikipedia)
• US Patent 5,465,627, Circularly magnetized non-contact torque sensor (Garshelis, Google Patents)
• EP 0803053, Circularly magnetized non-contact torque sensor (European Patent Office)
• Garshelis, Whitney and May, Development of a Non-Contact Torque Transducer for Electric Power Steering Systems, SAE 920707
• Klokholm and Aboaf, The saturation magnetostriction of thin polycrystalline films of iron, cobalt, and nickel (IBM Research)
• Patents by inventor Ivan J. Garshelis (Justia)