Release the bob and watch one complete swing before you time anything.
01Set the pendulum swinging in a small arc, and let it settle before you time anything.The formula T = 2π√(L/g) assumes a small angle. A wide swing has a measurably longer period, and one that’s still wobbling sideways isn’t a simple pendulum at all.
02Pick a fixed marker at the centre of the swing and watch that, not the bob.Your eye can judge the instant something crosses a fixed line far better than it can judge where a moving object is. In a real lab this is a pin or a mark on the bench behind the bob.
03Start the watch as the bob passes the centre, and say "zero" as you press.Zero, not one. Counting the release as "one" is the off-by-one that makes twenty oscillations out of nineteen and inflates g by five per cent.
04Count each time the bob returns to the centre travelling the SAME way.It crosses the centre twice per oscillation, once in each direction. Counting both is the other way to arrive at a period half what it should be.
05Stop on the twentieth complete oscillation and read the total.The total is a large, comfortable number that you read once. All the precision in your final answer comes from dividing it, not from the watch.
06Divide by the number of oscillations, and by the number you actually counted.If you lost count, say so and do it again. A total divided by the wrong count is a period that’s confidently and invisibly wrong.
07Repeat the whole timing and average.Reaction error is as likely to be early as late, so repeats pull it out. It’s the one error in this experiment that averaging actually helps with.