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(A) The resistivity of a conductor is given by $ho = \frac{m}{n e^{2} 	au}$,where $m$ is the mass of an electron,$e$ is the charge on an electron,$

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(A) The resistivity of a conductor is given by $ho = \frac{m}{n e^{2} 	au}$,where $m$ is the mass of an electron,$e$ is the charge on an electron,$n$ is the number of free electrons per unit volume,and $	au$ is the relaxation time.From this,we see that $ho \propto \frac{1}{	au}$.Also,the conductivity $\sigma$ is the reciprocal of resistivity,so $\sigma = \frac{1}{ho}$.Therefore,$\sigma \propto 	au$.The ratio of resistivity to conductivity is $\frac{ho}{\sigma} = \frac{ho}{1/ho} = ho^{2}$.Since $ho \propto \frac{1}{	au}$,we have $\frac{ho}{\sigma} \propto \frac{1}{	au^{2}}$.When the temperature of a conductor increases,the thermal velocity of electrons increases,which leads to more frequent collisions. Consequently,the relaxation time $	au$ decreases.Since $\frac{ho}{\sigma} \propto \frac{1}{	au^{2}}$,as $	au$ decreases,the ratio $\frac{ho}{\sigma}$ increases.

As the temperature of a conductor increases,its resistivity and conductivity change. The ratio of resistivity to conductivity

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