How can we measure the current in a circuit?

Two resistors of resistance $2 \,\Omega$ and $4 \,\Omega$ when connected to a battery will have:

An electric oven hot plate is connected to a $200 \, V$ line. It has two resistance coils $A$ and $B$,each of $30 \, \Omega$,which may be used separately,in series,or in parallel. Find the value of the current required in each of the three cases.

In the figure given below,$A, B$,and $C$ are three ammeters. The ammeter $B$ reads $0.5\, A$. (All the ammeters have negligible resistance.) Calculate:
$(i)$ the readings in the ammeters $A$ and $C$.
$(ii)$ the total resistance of the circuit.

The $V-I$ graphs for two resistors of the same material and the same radii with lengths $L_{1}$ and $L_{2}$ are shown below. If $L_{1} > L_{2}$, state with reason, which of these graphs represents voltage-current change for $L_{1}$?

What will happen to the current flowing through a conductor,if the potential difference is doubled?