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In a potentiometer, why is it necessaryto i) use a long wire ii) have uniform area of cross section of the wire and iii) use a driving cell whose emfis taken to be greater than the emfs of the primary cells?

i) To reduced potential gradient
ii) Uniform resistance and thereby uniform potential gradient
iii) Necessary condition to balance the bridge is secondary cell emf should be more than primary cell, otherwise the bridge can't be balanced.

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1. when the wire is longer , the potential gradient is smaller . smaller the potential gradient , more is the sensitivity of the potentiometer.
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2. a uniform wire will have a predictable level of resistance across its length ( resistance is in proportional to the cross sectional area ) . also the longer
it is ,  greater the degree of control you have over its resistance . lastly , a longer potentiometer can better dissipate heat. 
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3. the source of cell produces a potential difference across the potentiometer wire and this potential difference falls uniformly across the wire. when a cell of emf greater than the emf used , then it cannot be balanced on the wire as the wire will have a lower potential difference than the cell .
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