*Apparatus* : 1.5V (approx) cell, Resistance box, Push switch, Ammeter
(0-1A).

Set **R**at 10 W . Reduce in steps of 1 W , recording resistance and current. Read the Ammeter as accurately as possible. Release switch after each reading, otherwise the cell will run down during the experiment

- Repeat the readings, increasing
**R**back up to 10 W.Obtain average values of**I**, the current.

- Calculate 1/
**I**.

- Plot a graph of
**R**against 1/**I**. Draw the best possible straight line through the points - they will be quite scattered.

- The gradient of this graph is the emf of the cell.
The negative intercept on the y-axis is the internal resistance.

*Theory:*

The emf of a cell is the total p.d. it can produce around
the circuit, including any potential wasted in driving current through the
cell itself.

The internal resistance of a cell is simply the resistance from one terminal
of the cell to the other.

then **E = I (R+r)**

so **E/I = R + r**

Hence: **R = E(1/I) - r**

Therefore **R** plotted against
1/**I** has gradient** E**
and intercept -**r** .