**Superconductors:**

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.
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