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- Name an instrument that measures electric current in a circuit.

Define the unit of electric current. - What do the following symbols mean in circuit diagrams?

- An electric circuit consisting of a 0.5 m long nichrome wire XY

Define the unit of electric current.

**(a) **An instrument that measures electric current in a circuit is called an **"ammeter"**. The unit of electric current is **ampere **(A). 1 ampere is constituted by the flow of 1 coulomb of charge through any point in an electric circuit in 1 second.

(b) The following symbols mean in circuit diagrams are:

(i) Variable resistance or Rheostat.

(ii) Plug key or switch (closed).

(c) (I)

(c) (ii) Following graph was plotted between $V$ and $I$ values:

At potential difference $0.8\ V$,

$\frac {V}{I}=$\frac {0.8}{0.3}=$\frac {8}{3}$

At potential difference $1.2\ V$,

$\frac {V}{I}=$\frac {1.2}{0.45}=$\frac {8}{3}$

At potential difference $1.6\ V$,

$\frac {V}{I}=$\frac {1.6}{0.6}=$\frac {8}{3}$

**Conclusion: **If $I$ be the current through $XY$ resistor and $V$ be the potential difference across it, then the ratio $\frac {V}{I}=constant$.

$\Rightarrow V\propto I$, and Ohm's law is obeyed.