Refer to the video below for the setting up of the apparatus.

**Why do you need a variable resistor (rheostat)?**

Without the variable resistor, you will have only

__one set__of current I and potential difference V readings. Using the formula R = V/I, you are able to find the unknown resistor. But this method is__not so accurate__.Hence, to make it more accurate, we include a

__variable resistor to control the size of the current__through the circuit. Thus having__different readings of the potential difference V across the unknown resistor__.Instead of just one set of readings of I and V, we now have about 5 sets.

This allows us to

__plot a graph of V against__I.By finding the

__gradient of the best fit line__, we are able to find the resistance more accurately.*[gradient = V / I = R, hence the gradient of V-I graph represents resistance R]*

For** pure metallic conductor**, like the fixed resistor R, it obeys the **Ohm’s Law**, hence it is an ohmic conductor.

From the graph,** the current I flowing the conductor is **__directly proportional__** to potential difference V** across the conductor, provided physical conditions like temperature remains constant. *[the graph is a straight line with constant gradient, and passes through the origin]*

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