Magnetic Reluctance.
The reciprocal of permeance; magnetic resistance; the relative
resistance to the passage of lines of force offered by different
substances. The idea is derived from treating the magnetic circuit like
an electric one, and basing its action on magneto-motive force acting
through a circuit possessing magnetic reluctance.
Magnetic Reluctivity.
The reciprocal of magnetic permeability, q. v.
Synonym--Magnetic Resistance.
Magnetic Retentivity.
The property of steel or hard iron by which it slowly takes up and
slowly parts with a magnetic condition--traditionally (Daniell) called
coercitive force.
Magnetic Rotary Polarization.
If a plane polarized beam of light is sent through a transparent medium
in a magnetic field its plane of polarization is rotated, and this
phenomenon is denoted as above. (Compare Refraction, Electric, and see
Electro-magnetic Stress.) This has been made the basis of a method for
measuring current. A field of force varies with the current; the
polarization produced by such field is therefore proportional to the
current. (Becquerel & Rayleigh.)
A plane polarized beam of light passing through the transparent medium
in the magnetic field by the retardation or acceleration of one of its
circular components has its plane of polarization rotated as described.
The direction of the lines of force and the nature of the medium
determine the sense of the rotation; the amount depends upon the
intensity of the field resolved in the direction of the ray, and on the
thickness and nature of the medium.
Magnetic Saturation.
The maximum magnetic force which can be permanently imparted to a steel
bar. A bar may be magnetized beyond this point, but soon sinks to it.
The magnetism produced in a bar is prevented from depolarization by the
retentivity or coercive force of the bar. The higher the degree of
magnetization the greater the tendency to depolarization.
It is also defined as the maximum intensity of magnetism produced in a
paramagnetic substance by a magnetic field as far as affected by the
permeability of the substance in question. The more lines of force
passed through such a substance the lower is its residual permeability.
It is assumed that this becomes zero after a certain point, and then the
point of saturation is reached. After this point is reached the addition
of any lines of force is referred entirely to the field and not at all
to the permeability of the substance. But such a zero is only definable
approximately.
Magnetic Screen.
A box or case of soft iron, as thick as practicable, for protecting
bodies within it from the action of a magnetic field. The lines of force
to a great extent keep within the metal of the box on account of its
permeability, and but a comparatively few of them cross the space within
it.
Such screens are used to prevent watches from being magnetized, and are
a part of Sir William Thomson's Marine galvanometer.
A magnetic screen may be a sphere, an infinite or very large plane, or
of the shape of any equipotential surface.
Synonym--Magnetic Shield.