The resistance of platinum wire at 0$°\mathrm{C}$ is 22.05 $\mathrm{\Omega }$ and at 100$°\mathrm{C}$ it becomes 22.70 $\mathrm{\Omega }$. When the wire is heated to a temperature of t$°\mathrm{C}$ the resistance of wire becomes 24.91$\mathrm{\Omega }$. The value of t is 

1.  220

2.  440

3.  310

4.  550

The resistivity of iron is 1 × 10^–7 ohm – m. The resistance of iron wire of particular length and thickness is 1 ohm. If the length and the diameter of wire both are doubled, then the resistivity in ohm – m will be :

1. 1 × 10^–7

2. 2 × 10^–7

3. 4 × 10^–7

4. 8 × 10^–7

The resistivity of a wire :

The specific resistance of a wire is ρ, its volume is 3 m³ and its resistance is 3 ohms, then its length will be 

1. $\sqrt{\frac{1}{\rho }}$

2. $\frac{3}{\sqrt{\rho }}$

3. $\frac{1}{\rho }\sqrt{3}$

4. $\rho \sqrt{\frac{1}{3}}$ 

The specific resistance of all metals is most affected by :

1. Temperature

2. Pressure

3. Degree of illumination

4. Applied magnetic field