Determination of the acceleration due to gravity using the simple pendulum

If $l$ is the oscillating length and $T$ the period of oscillation, of the simple pendulum,

$$T=2\pi\sqrt{\frac{l}{g}}$$ $$T^{2}=(\frac{4\pi^{2}}{g})l$$

Gradient of the graph $T^{2}$ against $l = \frac{4\pi^{2}}{g}$

$$g=\frac{4\pi^{2}}{(gradient)}$$

1. Pass the pendulum string through the cork and suspend the pendulum from a stand.
2. Measure the oscillating length $l$ of the pendulum from the point of suspension down to the centre of the pendulum bob.
3. Note down the value of $l$ and fix the locating pin vertically below and close to the lowest point of the path of pendulum bob.
4. Pull the bob aside till it is inclined by a small angle to the vertical ($5^{\circ}$ or $6^{\circ}$). Release the pendulum to make it oscillate in a vertical plane and using the stop watch measure the time for 25 oscillations.
5. Record the time and repeat the experiment by increasing the value of $l$ by 10 cm after starting from about $l=40$ cm. Obtain about six recordings.