Resistance of a conductor increases with the increase in temperature.

Calculate the equivalent resistance between the points $A$ and $B$ in the following combination.

$(a)$ Name an instrument that measures potential difference between two points in a circuit. Define the unit of potential difference in terms of $SI$ unit of charge and work. Draw the circuit symbols for $(i)$ variable resistor,$(ii)$ a plug key which is closed.
$(b)$ Two electric circuits $(I)$ and $(II)$ are shown below.
$(i)$ Which of the two circuits has more resistance?
$(ii)$ Through which circuit does more current pass?
$(iii)$ In which circuit is the potential difference across each resistor equal?
$(iv)$ If $R_{1} > R_{2} > R_{3}$,in which circuit will more heat be produced in $R_{1}$ as compared to the other two resistors?

What is the process of depositing a layer of gold on silver called?

Electric current flows through three lamps when arranged in $(a)$ a series and $(b)$ a parallel circuit. If the filament of one lamp breaks,explain what happens to the other two lamps in both cases.

Nichrome is used to make the element of an electric heater. Why?