When the temperature of a conductor increases, the ratio of conductivity and resistivity _____

In the above circuit, if the current through $10 \mathrm{\Omega }$ resistor is $2.5 \mathrm{A}$, value of $R$ is ______.

A galvanometer of resistance $50 \mathrm{\Omega }$ giving full scale deflection for a current of $10$ milliampere is to be changed into a voltmeter of range $100 \mathrm{V}$.

A resistance of _____ $\mathrm{\Omega }$ has to be connected in series with the galvanometer.