Wires A and B have resistivities ${\rho }_A$ and ${\rho }_B$ , $\left({\rho }_B=2{\rho }_A\right)$ and have lengths $l_A$ and $l_B$ . If the diameter of the wire B is twice that of A and the two wires have same resistance, then $\frac{l_B}{l_A}$ is

In a conductor, if the number of conduction electrons per unit volume is $8.5\times {10}^{28}{\mathrm{m}}^{-3}$ and mean free time is $25f\mathrm{s}$ (femtosecond), its approximate resistivity is: $\left({\mathrm{m}}_{\mathrm{e}}=9.1\times {10}^{-31}\mathrm{k}\mathrm{g}\right)$

The Wheatstone bridge shown in the figure below, gets balanced when the carbon resistor used as $R_1$ has the colour code (orange, red, brown). The resistors $R_2$ and $R_4$ are $80 \mathrm{\Omega }$ and $40 \mathrm{\Omega }$, respectively. Assuming that the colour code for the carbon resistors gives their accurate values, the colour code for the carbon resistor, used as $R_3$, would be

A carbon resistance has the following colour code. What is the value of the resistance?