Weiss modified Langevin theory of paramagnetism by introducing a new concept of internal fields. Weiss assumed that in a real gas, the molecules are mutually influenced by their magnetic moments and consequently, there should exist within the gas a molecular field. This field, produced at any point by all the neighboring molecules, is proportional to and acting in the same sense as the intensity of magnetization. Let this internal molecular field be represented by H_i. Then
            Hi = λM
Here, λ is molecular field coefficient. M is the magnetization.
Then the effecting field is
            H_e = H + H_i = H + λM

According to Langevin’s theory of paramagnetism,
            M = nm² μ₀H / 3KT = μ₀M²_s H/3nkT
m = magnetic moment of each atoms,
n = number of atoms per unit volume,
H = magnetic field intensity,
K = Boltzmann’s constant,
T = absolute temperature,
Ms = nm = saturation value of magnetization.

Weiss replaced H by He. Therefore,
        M = = μ₀M2s (H + λM) /3nkT                            … (1)

. :    Volume susceptibility χm = M/H = μ0 nm2 (H + λM)/3kTH
i.e.,         χ_m = μ₀nm² /3kT = + λμ₀nm² /2kT χm
or         χ_m = (1 – λμ₀nm² /2kT) = μ₀nm² /3kT

. :        χ_m = (μ0nm² /3kT) = (μ₀nm² /3k) / T – λμ₀ nm² /(T - λμ₀ nm² /3k) = C/T – θ        … (2)

Eq.(2) is known as Curie Weiss law. The constant θ is known as Curie temperature. Curie Weiss law shows that below Curie temperature (T < θ), susceptibility becomes negative. However, it should be noted that for most of the paramagnetic substances, Curie temperature is quite low so that a situation for which T < θ is rare.

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