$(a)$ Name an instrument that measures electric current in a circuit. Define the unit of electric current.
$(b)$ What do the following symbols mean in circuit diagrams?
$(c)$ An electric circuit 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 was set up.
$(i)$ Draw a diagram of this electric circuit to study the relation between the potential difference maintained between the points $X$ and $Y$ and the electric current flowing through $XY$.
$(ii)$ The following graph was plotted between $V$ and $I$ values. What would be the values of $V/I$ ratios when the potential difference is $0.8 \ V$,$1.2 \ V$ and $1.6 \ V$ respectively? What conclusion do you draw from these values?

(N/A) An ammeter is used to measure electric current in an electric circuit.
One ampere is defined as the flow of one coulomb of electric charge through a circuit in one second.
$(b)$ $(i)$ Variable resistor (Rheostat) and $(ii)$ Closed plug key (switch).
$(c)$ $(i)$ The circuit diagram consists of a battery of four cells,an ammeter in series,a nichrome wire $XY$ in series,and a voltmeter connected in parallel across $XY$.
$(ii)$ Since the graph is a straight line passing through the origin,the ratio $V/I$ is constant,which represents the resistance $R$ of the nichrome wire.
From the graph,at $V = 1.6 \ V$,$I = 0.6 \ A$.
$R = V/I = 1.6 / 0.6 = 2.67 \ \Omega$.
Since the ratio $V/I$ is constant for all values $(0.8 \ V, 1.2 \ V, 1.6 \ V)$,it confirms Ohm's Law.