Silsbee rule: An important result of the existence of critical magnetic field is that there is also critical strength of current I_{c }flowing in the superconductor. Exceeding this limit also causes the disturbance of superconductivity. To derive the relation between critical current field consider a superconductor wire of radius r carrying a current I. This current will produce a magnetic field given by:

As discussed in the Meissner effect that one of the conditions of the superconducting state is that Magnetic flux density (B) = 0 inside the superconductors that is the magnetic flux cannot penetrate inside the superconductor. But experimentally it is not so. The magnetic flux does not suddenly drop to zero inside the surface. The phenomenon of flux penetration inside the superconductors was explained by H. London and F. London.

Derivation of London first equation:

Let n_{s} and v_{s} be the number density (number/volume) and velocity of superconducting electrons respectively. The equation of motion or acceleration of electrons in the superconducting state is given by

m(dv_{s}/dt) = -eE

or dv_{s}/dt = -eE/m (1)

where m is the mass of electrons and e is the charge on the electrons.

Depending upon their behavior in an external magnetic field, superconductors are divided into two types:

a) Type I superconductors and b) Type II superconductors

Let us discuss them one by one:

1) Type I superconductors:

a). Type I superconductors are those superconductors which loose their superconductivity very easily or abruptly when placed in the external magnetic field. As you can see from the graph of intensity of magnetization (M) versus applied magnetic field (H), when the Type I superconductor is placed in the magnetic field, it suddenly or easily looses its superconductivity at critical magnetic field (Hc) (point A).

After Hc, the Type I superconductor will become conductor.

b). Type I superconductors are also known as soft superconductors because of this reason that is they loose their superconductivity easily.

Superconductors are the materials whose conductivity tends to infinite as resistivity tends to zero at critical temperature (transition temperature).

Critical temperature (T_{c}): The temperature at which a conductor becomes a superconductor is known as critical temperature.

Critical Magnetic Field (Hc): The magnetic field required to convert the superconductor into a conductor is known as critical magnetic field.

Critical magnetic field is related with critical temperature as:

H_{c}(T) = H_{c}(0)[1 – T^{2}/T_{c}^{2}]

Meissner Effect:

Suppose there is a conductor placed in a magnetic field at temperature T (refer figure). Then the temperature is decreased till the critical temperature. See what happened (figure). Lines of force are expelled from the superconductor. This is called Meissner effect.

B is not 0 at T > Tc B=0 at T < Tc

Definition Meissner Effect: The expulsion of magnetic lines of force from a superconducting specimen when it is cooled below the critical temperature is called Meissner effect.

To prove that superconductors are diamagnetic by nature:

B is not 0 at T > Tc B=0 at T < Tc

As B = µ_{0} (H +M)

Where B is magnetic induction or magnetic flux density,

H is applied magnetic field or magnetic field intensity

And M is intensity of magnetization.

For superconductors B = 0

Thus either µ_{0} = 0 or H + M = 0

But µ_{0 }can not be zero,

Thus H + M =0

Or M = -H (1)

By definition of magnetic susceptibility

X = M/H

Put equation (1)

Thus X = -1

But magnetic susceptibility is negative for diamagnetic materials, thus it proves that superconductors are diamagnetic by nature.

Note: This article is referred from my authored book “Electrical Engineering Materials” having ISBN 8127234044.

A qualitative discussion of a successful theory of superconductivity was given by Bardeen, Copper and Schrieffer, known as BCS theory after the initials of their names. This theory accounts for all properties of superconductors.