Which particles are responsible for the flow of current in conductors?

In the circuit diagram shown,the two resistance wires $A$ and $B$ are of the same area of cross-section and same material,but $A$ is longer than $B$. Which ammeter,$A_{1}$ or $A_{2}$,will indicate a higher reading for current? Give a reason.

$A$ large number of free electrons are present in metals,yet no current flows in the absence of electric potential across it. Explain the statement with reason.

$(a)$ In the circuit shown, connect a nichrome wire of length $L$ between points $X$ and $Y$ and note the ammeter reading.
$(i)$ When this experiment is repeated by inserting another nichrome wire of the same thickness but twice the length $(2L)$, what changes are observed in the ammeter reading?
$(ii)$ State the changes that are observed in the ammeter reading if we double the area of cross-section without changing the length in the above experiment. Justify your answer in both cases.
$(b)$ "Potential difference between points $A$ and $B$ in an electric field is $1\, V$." Explain the statement.

Give reasons for the following:
$(i)$ Tungsten is used almost exclusively for the filament of electric lamps.
$(ii)$ Why do we use copper and aluminium wires for the transmission of electric current?

If the resistance of a conductor is doubled,the current is halved.