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Table Of Content
Definition
According to Einstein ‘Time is what a clock reads’.
Introduction
Any phenomenon that repeats itself regularly can serve as a measure of time. Human heart which beats regularly, rotation of earth around its axis, revolution of earth around the sun are some examples of repetitive phenomena’s, which serve as measures of time.
The most common unit of time is ‘second’. Originally, one second was defined as the time taken by a simple pendulum of length one meter in going from one extreme to the other extreme. Later on, this definition was modified from time to time. Eventually, one second used to be defined as the duration of 9,192,631,770 periods of radiation corresponding to impetrated transition between the two hyperfine levels (F = 4, M = 0 and F = 3, M = 0) of the ground state of Cesium-133 atom.
Atomic clocks are based on this definition. They ensure an accuracy of 1 in sec. i.e., an error of 1 sec. only in 5000 years.
Measurement of time intervals
The Italian physicist, Galileo was the first to attempt a simple method of measuring time. Galileo timed the oscillations of a lamp hanging from a long chain in a church using his own pulse as a watch. Later on, some other clocks developed. The various clocks for time measurement are as follows :
Electric Oscillator
The main a.c. supply in our country is 50Hz.The synchronous rotations of a motor running on a.c. can be used to obtain a time scale.
Electronic Oscillator
Vacuum tubes or junction transistors can be used for producing electromagnetic waves of high frequencies. The small time period of such oscillators can be used for measuring small time intervals.
Atomic clock
Time intervals of the order of Pico seconds are measured using atomic clocks. The radiations emitted by atoms have periods of the order of cesium clock (133) which was developed in 1964. They have an accuracy of 1 sec in every 1011 sec.
Quartz–crystal clock
A quartz crystal shows piezo-electric affect that is, if such a crystal is subjected to fluctuating mechanical pressure across its pair of faces, an oscillatory e.m.f. is developed across the other pair of perpendicular faces and vice-versa. The oscillations so produced can be used to measure time intervals.
Decay of elementary particles
By studying the decay of eliminatory particles, very small time intervals can be measured. Using Photographic emission techniques, measurement of time intervals ranging from 10-16 sec to 10-24 sec can be made.
Radioactive Dating
This technique is used to measure long time intervals. The basic principle of this technique is to calculate the time intervals by noting the ratio of the number of radioactive atoms which have undergone decay with the passage of time to the number of surviving atoms. Carbon dating is used to estimate the age of fossils, while uranium dating is employed for estimating the age of a rock or earth.
References
- Modern’s ABC of Physics (Vol. I)
- Pradeep’s Fundamental Physics (XI)