Clocks and watches
September 23rd, 2012
I have lately been fascinated by how clocks and watches work and here is a little summary of what I've found. Timekeeping devices of the current form started all the way back in the 13th century for astronomical and administrative purposes. Many technological advances have occurred since then but the essential idea remains the same. In the broadest of terms there are two main parts of a clock: a timekeeping source and an energy source. A pendulum, for example, is often used as a timekeeping source and the reason why it is an appropriate one is an underlying physical principle. A pendulum of a given length completes each oscillation in precisely the same amount of time. Therefore, by adjusting the length of the pendulum, it can be made to complete each oscillation in precisely 1 second. If we were living in a world where there was no friction and no air drag then that is all that was needed. One could theoretically design a mechanism which would count every oscillation of the pendulum and that would be the 'second' hand and one could design additional mechanisms to 'tick' once for every 60 'ticks' of the second hand which would be the 'minute' hand and so on and so forth.
But we live in a world with mechanical losses which means that the pendulum cannot go on oscillating indefinitely without providing additional energy. In the medieval ages I suppose one could have imagined hiring an underling to stand beside the pendulum and give it a 'kick' every so often but it's not a very feasible solution even for an outsourced economy! So we have had to find additional 'automatic' sources of energy. This additionally energy can be provided in different ways. In big old pendulum clocks this was often provided by a falling weight and in the small wrist watches this is done through a wound spring but the essential idea is the same. Since there is a small and finite amount of energy stored in a falling weight or a wound spring it is important to regulate how this energy is transferred to the timekeeping source. This is done through a neat little mechanism called an escapement mechanism which ensures that one doesn't have to wind the clock/watch or change the batteries too frequently.
The whole field of horology can, therefore, be summarized in a few statements. The effort is to find a phenomenon in nature which is periodic and consistent and to keep it going by providing additional energy. Obviously the mechanical implementation is intricate fascinating and beautiful but that is the essence. Wrist watches obviously don't have oscillating pendulums but they have a similar timekeeping source - a mass rotating on a spring. Modern wrist-watches have another more accurate timekeeping source - a tuning fork made of a quartz crystal designed to oscillate at precisely 32,768 times a second and the energy comes from a battery. Atomic clocks have a yet more precise timekeeping source: atoms transitioning from one energy state to another emit radiation at a precise frequency. Since that frequency is constant it may be used as a timekeeping source. Atomic clocks based on Caesium-133 count 9192631770 cycles of the radiation emitted by the energy transition of the Caesium atom and the time it takes for those cycles to complete is equal to 1 second.
Following are some additional references which show the actual mechanisms of a clock:
 http://electronics.howstuffworks.com/gadgets/clocks-watches/clock.htm
http://www.youtube.com/watch?v=MDYjUDrCPv0&feature=g-like
Thoughts on buying a watch:
