State which has a higher resistance,a $50 \, W$ or a $25 \, W$ lamp bulb,and by how many times?

$(a)$ What is meant by saying that the potential difference between two points is $1$ volt?
$(b)$ Why does the connecting cord of an electrical heater not glow while the heating element does?
$(c)$ Electrical resistivities of some substances at $20^{\circ}C$ are given below:

Silver	$1.60 \times 10^{-8} \, \Omega m$
Copper	$1.62 \times 10^{-8} \, \Omega m$
Tungsten	$5.2 \times 10^{-8} \, \Omega m$
Iron	$10.0 \times 10^{-8} \, \Omega m$
Mercury	$94.0 \times 10^{-8} \, \Omega m$
Nichrome	$100 \times 10^{-6} \, \Omega m$

Answer the following questions in relation to them:
$(i)$ Among silver and copper, which one is a better conductor? Why?
$(ii)$ Which material would you advise to use in electrical heating devices? Why?

An electron in a metal is free to move anywhere but cannot leave the metal.

The equivalent resistance between points $A$ and $B$ in the following electric circuit is:

$B_1, B_2$ and $B_3$ are three identical bulbs connected as shown in the figure. When all three bulbs glow,a current of $3\,A$ is recorded by the ammeter $A$.
$(i)$ What happens to the glow of the other two bulbs when the bulb $B_1$ gets fused?
$(ii)$ What happens to the reading of $A_1, A_2, A_3$ and $A$ when the bulb $B_2$ gets fused?
$(iii)$ How much power is dissipated in the circuit when all three bulbs glow together?

How many microamperes $(\mu A)$ are equal to $1$ milliampere $(mA)$?