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