The twisting of a ferromagnetic rod through which an
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
is flowing when the rod is placed in a longitudinal magnetic field. It was discovered by the German physicist
Gustav Wiedemann
Gustav Heinrich Wiedemann (; 2 October 1826 – 24 March 1899) was a German physicist and scientific author.
Life
Wiedemann was born in Berlin the son of a merchant who died two years later. Following the death of his mother in 1842 he lived w ...
in 1858
. The Wiedemann effect is one of the manifestations of
magnetostriction
Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field chan ...
in a field formed by the combination of a longitudinal magnetic field and a circular magnetic field that is created by an electric current. If the electric current (or the magnetic field) is alternating, the rod will begin torsional oscillation.
In linear approach angle of rod torsion ''α'' does not depend on its cross-section form and is defined only by current density and magnetoelastic properties of the rod:
:
,
where
*
is
current density;
*
is magnetoelastic parameter, proportional to longitudinal magnetic field value;
*
is the
shear modulus
In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain:
:G \ \stackre ...
.
Applications
Magnetostrictive position sensors use the Wiedemann effect to excite an ultrasonic pulse. Typically a small magnet is used to mark a position along a magnetostrictive wire. The magnetic field from a short current pulse in the wire combined with that from the position magnet excites the ultrasonic pulse. The time required for this pulse to travel from the point of excitation to a pickup at the end of the wire gives the position. Reflections from the other end of the wire could lead to disturbances. In order to avoid this the wire is connected to a mechanical damper that end.
See also
*
Matteucci effect the inverse effect
*
Magnetostriction
Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field chan ...
*
Magnetomechanical effects for a collection of similar effects
References
{{reflist
Magnetism
Magnetic ordering