How we can select the resistance of the driver cell and length of the wire for a potentiometer?

Potentiometer is a device used to measure the internal resistance of a cell, to compare the e.m.f. of two cells and potential difference across a resistor. It consists of a long wire of uniform cross sectional area and of 10 m in length. The material of wire should have a high resistivity and low temperature coefficient.  The wires are stretched parallel to each other on a wooden board. The wires are joined in series by using thick copper strips.
Potential gradient is given by,
( σ = V / L )
As length of the wire (L) is more, small will be the magnitude  of σ  and we  have more accuracy in measurement of potential difference with the help of potentiometer