For those who do not know what Master Clocks are I will provide some information on their use and construction. Otherwise select from the links above or on the right, for photos and descriptions of that particular model.

 

Firstly to be truly accurate a clock strikes or chimes the hours, whilst a timepiece has no chime or any audible means of indicating the hours.

The use of master timepieces, referred to from now on as master clocks, probably reached its peak in the first half of the 20th century, and although they have declined in their use today, examples still abound, although they are usually of a modern electronic type.

Imagine a large office or factory complex where they all need to work to exactly the same time. The simple way to achieve this was to have one central clock. This central clock, or master clock, would then send the time out to various small clocks, called slave clocks, throughout the factory or office. If the master clock was 2 minutes slow then all the clocks were slow. So if you adjust the master clock by advancing it by 2 mins, then all the slave clocks would automatically keep in time and be advanced as well. A tremendous time saver if you had 500 slave clocks!

The master clock design was invariably a pendulum unit that beat at 1 second, 3/4 second, or 0.5 second intervals, although after the mid 1980's they were in decline as silicon gradually took their place.
These initially were housed in ornate wooden cases with a glass front door, and often, though not always, with a dial mounted in the top half of the case, much like a conventional wall clock.

They were almost always electrically powered and they communicated with their slave dials by sending low voltage electrical pulses to them at the correct interval. The slave dials usually have an electric solenoid in them, which when activated by the pulse from the master clock, moved the hands around. The United Kingdom favoured a single pulse to be sent out from the master to the slave once every minute or every 30 seconds, depending on each particular maker of the system. Whereas continental systems favoured an alternating pulse to be sent every minute to the slave. Obviously the correct type of slave has to be used with its respective system. Often clocks made by one maker for his own master system will work with another makers master clock - provided they both use the same system.

NONE of these slaves should ever be connected to the mains - they don't like it, and there's a reasonable chance you won't either!

Why not a good old wind-up system instead of electricity, I hear you say? Well wind-up clocks are never really very accurate unless expensive fusee movements are used (see here)  and if you employed a more accurate weight driven clock you must remember to wind it. And in premises that may be empty weekends and evenings, sooner or later the inevitable will happen, and the clock will stop.
As accuracy was another important factor, specialised electric master clocks evolved. These were often powered from batteries which were trickle charged from the mains. This meant they could run continuously, even in power strikes, and because of their accuracy they could be left unattended for long periods of time.

Some systems employed electric magnets to keep the pendulum swinging, while others had mechanical means of occasionally impulsing the pendulum. In fact International Time Recorders, later to come under IBM, used a spring wound clock (despite my comments above) but in this instance they were wound regularly by electricity, thus avoiding the inherent accuracy problems that spring clocks normally have.
Another feature of these clocks is that they were often built to a high standard of external finish. The clocks were an expensive investment and often took pride of place somewhere where visitors to the company would notice them. The more expensive and accurate clock models, costing in the region of up to a thousand pounds in today's money.

Prior to the advent of atomic time any institution that needed spot on accuracy like the BBC, Post Office Telephones, Hospitals for example, would employ a high quality master and slave system. Amazingly though, they would only keep time to the same standard as an ordinary quartz digital watch does today!
Top quality time keeping like that required by the Royal Observatory at Greenwich, and other observatories world wide, employed an even more specialised type of master that was contained in a vacuum and housed in a special temperature controlled environment. These it has to be said cannot be matched by modern day watches, and prior to the arrival of atomic clocks were the most accurate clocks mankind has ever made.