$(a)$ Name an instrument that measures electric current in a circuit. Define the unit of electric current.
$(b)$ What do the following symbols mean in an electric circuit?
$(c)$ Draw a closed circuit diagram consisting of a $0.5\, m$ long nichrome wire $XY$,an ammeter,a voltmeter,four cells of $1.5\, V$ each,and a plug key.

(N/A) The instrument used to measure electric current in a circuit is an ammeter.
The unit of electric current is the ampere $(A)$. One ampere is defined as the flow of $1$ coulomb of electric charge through a cross-section of a conductor in $1$ second.
$(b)$ $(i)$ The symbol represents a rheostat or variable resistance.
$(ii)$ The symbol represents a closed plug key or a closed switch.
$(c)$ The circuit diagram consists of a battery of four cells (total $6\, V$),a plug key (closed),an ammeter connected in series with the $0.5\, m$ nichrome wire $XY$,and a voltmeter connected in parallel across the wire $XY$.