To Study The Variation in Potential Drop With Length of a Wire For a Steady Current
To study the variation in potential drop with length of a wire for a steady current.

Apparatus and material
Apparatus. Potentiometer:
Material: A fully charged 4.5 V battery or battery eliminator, a low resistance rheostat, a voltmeter of range (0-3.0 V), an ammeter (0-3) A, a one way key, a jockey, a set square, connecting wires and a piece of sand paper.

For a potentiometer with wire of uniform material density and thickness (cross-sectional area) carrying a steady current, potential drop is proportional to the length of the wire.
where K is the drop of potential per unit length. It is called the potential gradient.



  1. Draw a circuit diagram showing the scheme of connections as in figure.
  2. Remove the insulation from the ends of the connecting copper wires with a sand paper.
  3. Connect the positive pole of the battery (eliminator) (a battery of constant e.m.f.) to the zero end (P) of the potentiometer and the negative pole through a one-way key, an ammeter and a low resistance rheostat to the other end (Q) of the potentiometer.
  4. Connect the positive terminal of the voltmeter to the end P of the potentiometer and the negative terminal to the jockey.
  5. Touch the end of the jockey to the end Q of the potentiometer.
  6. Close the key and set the rheostat such that the voltmeter gives full scale deflection (3 V).
  7. Touch the jockey at end P at 0 (zero) cm. The voltmeter will give zero deflection.
  8. Touch the jockey at marks separated by 50 cm length of wire. Note the voltmeter reading in each case.
  9. Record your observations in tabular form as given ahead.

Observations and  Calculations

Calculation from  graph
Plot a graph choosing a suitable scale, for the values of potential drop V along y-axis and length l  along x-axis as shown in figure.