1.4 Measurement of Time

The measurement of time started with the invention of sundials by the ancient Egyptians somewhere around 1500 B.C. 

Sundial was an instrument used to measure the time by the shadow of the pointer cast by the sun.

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A clock or watch measures long periods of time, while a stopwatch measures short times.

The smaller units of time are centiseconds, milliseconds, microseconds and nanoseconds.

Very short times (fractions of a second) are measured by using electronic devices that should start and stop automatically with sensors.

Devices Measuring Time

1. A clock or a watch measures long periods of time.
2. Short time intervals are measured by using a stopwatch.
3. Very short times (fractions of a second) are measured using electronic devices that should start and stop automatically by sensors.

Measurement of Short Interval of Time 

Very short time intervals are measured by electronic timers, which should start and stop automatically.

Without electronic timers, a short time interval can be measured mechanically using a simple

pendulum experiment.

Apparatus

 To do this experiment, a retort stand, clamps, a pendulum bob, a metre rule, a stopwatch and a long piece of string are used.

Time Period

The time required to complete one vibration is called the time period of vibrating a body.

It is denoted by “T” and is measured in seconds.

Time Period of a Pendulum

A small body of a mass suspended by a string from a fixed support is known as a pendulum. The string should be weightless and inextensible.

Procedure

1. Attach a measured length “ l ” of string from the centre of the bob to the hanging point.
2. Set the pendulum to swing and record one oscillation from one end to the other end and coming back.
3. When the bob is at one end, start your stopwatch and count 20 complete vibrations and record the time taken by these oscillations.
4. For accuracy, repeat the experiment for another 20 vibrations and take the average.
5. The time for one pendulum cycle is calculated using the following formula.
6. Repeat this experiment for different pendulum lengths, say 40cm, 60cm, 80cm and 100cm.