(a) Name an instrument that measures electric current in a curcuit. Define the unit of eletric current.
(b) What do the following symbols, (Fig. 3.58) mean in a circuit diagram ?
( 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) Graph shown in (Fig. 3.59) was plotted `V and I` values.
What would be the values of `v//I` ratios when the potential differences are `0.8 V, 1.2 V and 1.6 V` respectively ? What conclusion do you draw from these values ?
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Explanation

(a) Electric current is measured by an instrument called an ammeter.
The unit of electric current is ampere. When 1 coulomb of charge flows through any cross-section of a conductor in 1 second, the electric current flowing through it is said to be 1 ampere
(b) (i) This symbol represents a variable resistance or rheostat
(ii) Thus symbol represents on plug key or switch which is closed.
(c) (i) The circuit diagram to study the relation between V and I is shown in (Fig. 4).
(ii) When `V_1 = 0.8 V, I_1 = 0.3 A` Thus, `(V_1)/(I_1) = (0.8 V)/(0.3 A) = 2.67 Omega`
When `V_2 = 1.2 V, I_2 = 0.45 A` Thus, `(V_2)/(I_2) = (1.2 V)/(0.45 A) = (120)/(45) Omega = (8)/(3) Omega = 2.67 Omega`
When `V_3 = 1.6 V, I_3 = 0.6 A`. Thus, `(V_3)/(I_3) = (1.6 V)/(0.6 A) = (16)/(6) Omega (8)/(3) Omega = 2.67 Omega`
it is clear that `(V_1)/(I_1)=(V_2)/(I_2)=(V_3)/(I_3)` or `(V)/(I)` = constant.
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