48
3 Effects of the Introduction of Superconductivity into Electromagnetism
materials. This is caused by the fact that the magnetic moment is not produced by M
but by H. The magnetization of a superconductor is given by
M =
B
μ 0
− H 0 ,
(3.49)
when there is no influence of demagnetization caused by the shape, where B is
the mean value of the magnetic flux density inside the superconductor and H 0 is the
external magnetic field. The magnetization defined by (3.49) is not a local quantity,
as defined in (3.47), but is a macroscopic quantity for each superconductor. This is
also different from the mean value of (3.47). This description for the superconductor
is attributed to the treatment by thermodynamics for long period, beginning after the
discovery of superconductivity, in which the inner magnetic flux density B was
discussed as an internal variable against the external variable H 0 . This also means
that the superconductor has not been treated in electromagnetism.
The analogous comparison in the electric phenomena is electrostatic shielding
in a conductor and electric polarization in a dielectric material. These are similar
shielding phenomena, although the shielding is imperfect in the dielectric material,
while the shielding is perfect in the conductor. Both of these shielding phenomena
are explained as due to relative movement of positive and negative electric charges
under the applied electric field, although these phenomena have not been compared.
The terms are also different, i.e., electrostatic shielding and electric polarization. For
this reason, there was no confusion as in the magnetic phenomena. If we discuss
these phenomena as in the magnetization, however, we have P = 0 in a conductor,
assuming the relationship of D = 0 E. On the other hand, it is true that the electric
dipole moment in the conductor is not zero under the condition of electrostatic
shielding. Thus, a similar contradiction appears in electric phenomena.
When we look back at the magnetization, it makes a difference whether the source
of the magnetic moment is a real current that produces H or a virtual magnetization
current that produces M. In addition, the magnetization in a superconductor and that
in a magnetic material are also different from the viewpoint of shielding. Nevertheless, the same term is used for these phenomena. For this reason, some scientists have
believed that the magnetization in a superconductor can be described either by H or
by M. From the consideration of a hollow superconductor, as in Fig. 3.11, however,
it is clear which is correct.
As shown above, the scientific description of magnetization is not complete.
The introduction of superconductivity has clarified this lack of completeness. The
complete description cannot be implemented by the author alone, since the agreement of society is required. Here, the author wishes to propose the following: Since
the term “magnetization” has been used for magnetic materials for long time, this
term may continue to be used for these materials. Then, we need to use another term
for superconductors. The first proposal is to use “magnetic moment density”, which
follows from the original definition of magnetization, i.e., the magnetic moment in a
unit volume. In addition, from the basic viewpoint of the E-B analogy, it is desirable
3 Effects of the Introduction of Superconductivity into Electromagnetism
materials. This is caused by the fact that the magnetic moment is not produced by M
but by H. The magnetization of a superconductor is given by
M =
B
μ 0
− H 0 ,
(3.49)
when there is no influence of demagnetization caused by the shape, where B is
the mean value of the magnetic flux density inside the superconductor and H 0 is the
external magnetic field. The magnetization defined by (3.49) is not a local quantity,
as defined in (3.47), but is a macroscopic quantity for each superconductor. This is
also different from the mean value of (3.47). This description for the superconductor
is attributed to the treatment by thermodynamics for long period, beginning after the
discovery of superconductivity, in which the inner magnetic flux density B was
discussed as an internal variable against the external variable H 0 . This also means
that the superconductor has not been treated in electromagnetism.
The analogous comparison in the electric phenomena is electrostatic shielding
in a conductor and electric polarization in a dielectric material. These are similar
shielding phenomena, although the shielding is imperfect in the dielectric material,
while the shielding is perfect in the conductor. Both of these shielding phenomena
are explained as due to relative movement of positive and negative electric charges
under the applied electric field, although these phenomena have not been compared.
The terms are also different, i.e., electrostatic shielding and electric polarization. For
this reason, there was no confusion as in the magnetic phenomena. If we discuss
these phenomena as in the magnetization, however, we have P = 0 in a conductor,
assuming the relationship of D = 0 E. On the other hand, it is true that the electric
dipole moment in the conductor is not zero under the condition of electrostatic
shielding. Thus, a similar contradiction appears in electric phenomena.
When we look back at the magnetization, it makes a difference whether the source
of the magnetic moment is a real current that produces H or a virtual magnetization
current that produces M. In addition, the magnetization in a superconductor and that
in a magnetic material are also different from the viewpoint of shielding. Nevertheless, the same term is used for these phenomena. For this reason, some scientists have
believed that the magnetization in a superconductor can be described either by H or
by M. From the consideration of a hollow superconductor, as in Fig. 3.11, however,
it is clear which is correct.
As shown above, the scientific description of magnetization is not complete.
The introduction of superconductivity has clarified this lack of completeness. The
complete description cannot be implemented by the author alone, since the agreement of society is required. Here, the author wishes to propose the following: Since
the term “magnetization” has been used for magnetic materials for long time, this
term may continue to be used for these materials. Then, we need to use another term
for superconductors. The first proposal is to use “magnetic moment density”, which
follows from the original definition of magnetization, i.e., the magnetic moment in a
unit volume. In addition, from the basic viewpoint of the E-B analogy, it is desirable