Experiments have hove shown that the insulation resistance of the insulators decreases rapidly with the increase in temperature following the logarithmic curve, If R₁ is the insulation resistance at t₁⁰C, then its insulation resistance R₂ at t₂⁰C is given by

Log_e R' ₂ = Log_e  R₁' -K(t₂-t₁)

Log_e R_{1 '}= K (t₂-t₁) where K is a constant, whose value is determined form the initial conditions.
       R₂
If the insulation resistance becomes 1 of the resistance at the initial temperature for a rise of x⁰C then
                                                    n
by substituting R1'  = n and t₂-t₁=x in above expression
                      R2'
we get log_e n=Kx

or            K = 1  log_{e n}
                     x

Now the insulation resistance for any rise of temperature (say, t) can be determined form the relation

loge R₂' = loge R'₁-t  log_e n
                           x

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