Definition of time
The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of ground state of the Cesium 133 atom.
That’s lovely, but what does it mean? Hang on tight, we’ll come to that. But before that, let’s go back in history, few thousand years. To the land of mighty Egyptians. Yeah, you just thought of Pyramids and the movie Mummy, didn’t you? Yeah, moving along!
Sun to the rescue
Timekeeping became a necessity when humans started being social. As with everything on earth, it all started with Sun. Being the source of a vast amount of energy in the form of light, what better way of tracking time than to use that light.
Egyptians - the timemasters
- Obelisks: An Egyptian invention. Shadows are tracked to calculate the time of day.
- Sundial: A device that uses shadow cast by sunlight to measure time as the sun moves overhead. Egyptians used these in B.C. 3500. In 1500 B.C. Egyptians built a more accurate sundial which was divided into 10 parts. This one was accurate only half the time and afternoon it had to be rotated by 180°
- Path of sun changes throughout the year.
- Doesn’t work on rainy or overcast days.
- Doesn’t work during the night.
- Water Clocks: Used mostly as time measuring devices.
- First mechanical clocks might have been used in monasteries to keep monks aware of the prayer times.
- Most of these were large weight driven “turret” clocks which stuck at every hour and did not have a face or a hand.
- Salisbury Cathedral in England has the oldest clock which chimes every hour. These type of clocks had errors of up to half an hour a day and weren’t accurate.
- During mid of the 1400s, German and Italian clockmakers started making clocks that made use of wound spring. Most of these had only hour hands and the minute hand came into existence only during the 1650s.
- Master inventor Galileo devised the timekeeping property of pendulum and in 1656, Christiaan Huygens started using the pendulum as a time controller in clocks.
- The time period of a pendulum was independent of the mass of the pendulum and to the extent of the swing, but depended only on the length of the pendulum. A pendulum of around 99 cm (39 inches) has a time period of around 1 sec. Even an increase in length by 0.025 mm would make a difference of 1 sec in a day. Which means a steel pendulum can lose up to a second a day with a temperature variation of 2.2°C. A special alloy called “Invar” which has a small co-efficient of expansion is sometimes used to make pendulums.
- A series of clocks which were weight driven and had pendulums were devised and built. Due to their weight they were not suitable for hanging from the wall, instead, the casing was extended towards the ground and this led to the creation of “Grandfather” clocks.
- In 1670 the seconds pendulum was added to the clock which oscillated once every second.
- An escape wheel is used which is allowed to rotate one tooth per double swing of the pendulum and also transmits the impulse to the pendulum.
- Escapement has two main functions: transmit the impulse without hampering the free swing and to transmit a uniform impulse.
- Edmund Beckett invented a gravity clock using double three-legged gravity escapement and used this for Westminster which we adorably call The Big Ben .
- Escapement: Escapement is used to transfer the gravitational force acting on the weights to the clock’s mechanism. In this mechanism, a weight unwinds a rope from a barrel which turns the toothed escape wheel. A vertical rod with pellets at each end called a verge is used. When the weight drops and the wheel rotates, the top pallet arrests the wheel resulting in oscillation on the regulating weights. This oscillation turns the verge and that releases the wheel from the pellet until it is caught by the second pellet on the other side of the verge. This process is repeated. This action produces the “tick-tock”.
- The wheelwork: This mechanism, also called the train is responsible for transmitting the motion from the weight or spring mechanism to the hands of the clock through a series of wheels and pinions precisely calibrated to ensure steady transmission of power. There is a gearing mechanism that drives the clock and uses a 12:1 gearing ratio to move the hour hand.
- First made in 1840, the first electric clock had a pendulum and spring mechanism and used an electrical impulse to operate a number of dials.
- The first complete electric clock was invented in 1906.
- Quartz crystal was first used in clocks in 1929. A big leap in precise timekeeping.
- A ring of quartz of 2.5 mm in diameter is suspended by threads in a heat-resistant chamber. Electrodes are connected to the surface of the ring and the 100,000 hertz frequency of vibration is reduced by a process called frequency division and applied to a synchronous motor.
- 6 million to 1 ratio of gearing is used to move the second hand and the precision is of the order of one second every 10 years.
- Watches came into use around the 1500s. Peter Henlein, a locksmith from Germany made early models of watches using verge as the escapement method.
- Germany and France produced early watches which were not wrist watches but were worn on chains around the neck.
- These early watches had no minute or second hands and had only hour hands.
- Early watches used wound up springs, which were coil springs that could be wound up to store energy. This energy is then transferred using wheel train and escapement.
- The major drawback was the change in torque as the spring unwinds. This was countered by use of fusee, a cone-shaped, grooved pulley system that kept the torque constant by the variable diameter of the cone.
Navigation and need for precision
Navigators needed to have accurate ways of determining the time to also determine their location.
- Latitude could be calculated based on the position of sun’s angle at noon.
- Time at a reference point was necessary to identify the longitude. At equator 15° change in latitude is equivalent to an hours difference in time.
- Pendulums do not work well at sea.
- In 1714, the Engish government promised a reward of £20,000 to anyone who can measure longitude to the accuracy of 0.5° which corresponds to an error of 35 miles at the equator and an accuracy of 3 s/day.
- The prize was won by John Harrison who made the sea going H4 clocks with the accuracy of 0.2 s/day with the error of only 30 km on a 7-day trip.
Greenwich Mean Time
- Set as a standard in 1884.
- Since the path of the sun varies, there was a deviation of +/- 15 mins. Hence mean annual average of motion of sun was considered.
- 1 second = 1/864,000 of a mean day.
- Variation in earth’s rotation.
- Replaced by UTC (Coordinated Universal Time) which is pretty much similar to GMT and is maintained by an ensemble of around 260 clocks maintained in 40 countries.
Atomic clocks were a major breakthrough in the development of highly accurate clocks for use in various scientific devices and in modern exploration including space.
- 1955: Cesium atomic clock was invented by Essen and Parry at National Physical Laboratory.
- More clocks were reproduced in the following years and Caesium clocks were adopted as the basis for time measurement.
- Present day atomic clocks have an accuracy of 1 sec in 3 billion years.
Working of atomic clock
- Uses quantum mechanical states of energy and precise energy levels of electrons.
- The frequency at which Cesium releases peak energy is calculated and time is calculated accordingly.
- High precision. Used in space technology where nanosecond level of precision is required.
Awesome podcasts hosted by our friends that can help with productivity!
Image: Shankar’s Instagram