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Timekeeping – Clocks

The pendulum clock

The speed of the clock is the only thing that controls timekeeping. Adjusting the pendulum will make the clock go faster or slower. All other factors can be considered constant. This can be seen from the equation relating to the period of swing of a simple pendulum.

T = 2π√(L/g)


  • T is the period in seconds (s)
  • pi = 3.14 (it is also written as the Greek letter π)
  • L is the length pendulum in meters or feet
  • g is the acceleration due to gravity (9.8 m/s² or 32 ft/s²)

Lengthen pendulum to go slower and vice versa

If your antique clocks are correctly set up and are fast and gaining time, you will need to lengthen the pendulums. If your clock is slow, you will need to shorten the pendulum of the clock, to quicken it up.

All clock pendulum rods and bobs prior to the temperature compensated pendulums either will expand or contract with temperature changes. They need to be adjusted accordingly. Most antique clocks in centrally heated houses will be re-markedly accurate though, after you have adjusted to your mean temperature.

Temperature Compensated Pendulums on Clocks

Probably the earliest method for trying to overcome deviations in timekeeping as a result of temperature changes was the wood rod pendulum. In long grain the wood rod will expand only a little.This expansion will be compensated by the addition of a large brass cased lead bob. This brass bob will also expand slightly up and down from the rating nut, with temperature changes.

Every Increasing Accuracy

Refining the accuracy of the very best precision clocks started from the early 18th century with Harrison’s grid iron pendulum. Differing expansions of metals was understood many years earlier. Harrison devised a pendulum with a specific proportion of brass and iron. These two metals would have rates of expansion and contraction that would effectively cancel each other out.

George Graham

In 1722 Graham produced a mercury compensated pendulum. The mercury as seen in a regulator clock in the picture above is contained in a jar. This is effectively acting the same as a normal brass pendulum bob. Mercury will expand roughly 6 times the rate of steel and so in the ratio 1:6. The expansions will roughly cancel each other out. The thermal coefficients of the differing metals is therefore important when building compensated pendulums. Glass being a poor conductor of heat was the only real negative to this invention but this method proved very effective and the pendulums do look stunning. These type of clocks are very collectible.

John Ellicott

There were various other pendulum’s developed like Ellicott’s compensated pendulum and Richie’s compensated pendulum on regulator clocks.Then the eventual use of metals like Invar which is a mix of nickel/iron and small quantities of carbon and manganese.


All these compensated pendulums are just trying to keep the effective length of the pendulum the same. When I say effective length of the pendulum, this will be from the bending point of the feather at the top of the clocks pendulum, to the centre of gravity of the pendulum near its bottom.  This is why on some clocks coins or small weights were added on the rod to change fine timekeeping. I will happily answer more question if you contact me.

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How to spot fakes – a brief guide

Buying Clocks Advice

So you have finally managed to take the plunge and purchase your antique clock. There are many antique clocks available on the market but not all are original examples. There is nothing worse than spending lots of hard earned money on something that you later find out to be not what you originally thought. Regularly you will only find out something is amiss when someone knowledgeable visits your home. I have had to break bad news when asked for my comments on many occasions.

When buying an antique clock to purchase from someone you can trust. Finding someone like this is never easy. Try and find someone who is long established and are specialists in this field. Maybe they will offer you a money back guarantee. You should then have peace of mind.

Some restoration will have been carried out on all antique clocks. This should be sympathetic restoration though and not major surgery. I have devised some basic things to look out for below.

Genuine London Antique Clock C.1770

Look. Smell. Feel.

  • Probably the most common part of an antique grandfather clock to have been replaced is the base of the clock. Clocks used to stand on cold, damp floors, and many bases simply rotted away or were attacked by the dreaded woodworm. Clearly the bottom feet or plinth is an acceptable part of restoration but not the entire base section. Easy signs of new bases are when the wood does not match the trunk door. If the figuring of the wood and colour is different be-ware.

    On London and south country clocks the backboard should be old and full length. If the backboard is rotted at the bottom and stops way short of the bottom, this is something to be careful of. If the backboard has rotted away, how much of the base has been rebuilt? In North country clocks, backboards can be made in two pieces from the later part of the 18th century.

  • Style Issues

  • Movements have sometimes been replaced. Make sure the case style corresponds to the makers location. i.e. London mahogany clocks do not have swan neck pediments but dome or pagoda tops. You can look at some good reference clock books for information on what clock cases should look like for different areas of the country. Each area of the UK in the 18th century had a distinctly different style of cabinet feature. For instance London clocks do not use simple oak cases but these are either veneered in walnut, ebony, mahogany or decorated with chinoiserie.
  • Look for Spare Holes

  • Does the dial belongs to the movement? Are there any spare holes in the front plate where another dial has been ? If the clock has a brass dial, this is attached directly to the front plate. Not by means of an iron false plate. (as used on white painted dial clocks) The winding holes on ‘8-day’ clocks are well placed within the centre of the dial These do not spoil the engraving or chapter ring.
  • Dial Features

  • If the clock is an ‘8-day’ example that the clock has a second hand. 99% of all ‘8 day’ clocks should have a second hand to just below the 12 0’clock position. If there is an obvious reason why a second hand cannot be fitted like a ‘penny moon’ feature then this is OK. As a rule though ’30Hr’ clocks do not usually have second hands, and so if the clock has had a later ‘8-day’ movement fitted this is why you should wonder why there is no second hand.
  • Does the age of the movement corresponds to the age of the cabinet? All dials have dating features, for instance inside quarter divisions, type of spandrels. You can date these very easily with good reference books. English walnut cases date from the 17th century up to about 1760. Mahogany cabinets date from about 1750 onwards. Oak cabinets tend to run straight through the 17th and 18th and 19th century’s.
  • Any Packing Under Seatboard?

  • Look for any obvious packing under the seat-board that cannot be accounted for? If the seat-board is old and warped ? A small piece of packing may be necessary.
  • Prior to 1820 all English antique clocks movements should strike on a bell and not a gong. Gong striking is popular around C1900.
  • Look for rub marks from where the weights and pendulum have banged over the years. If you see 1 smooth rub on inside under trunk door and your clock has 2 weights, this is not a good sign. If there are marks on backboard far away from where pendulum is hanging, and the clock is keeping time. Ask what would have caused these marks.

    If the pendulum feather has been changed for a stiffer example, the pendulum height can change slightly This should only be a slight change.

  • Fitting Issues?

  • Investigate whether the dial fits the mask correctly and there are no large gaps. The size of glass should be similar to the size of the dial.
  • Is the face made from 1 piece of brass? This brass prior to C1800 will be cast brass and so thick and thin. The arch section of the dial should be from the same section of brass. If a square dial is later converted to an arch dial case, this is why some dials are made of two pieces.
  • Carved Cases?

  • Finally I have never seen an original fully carved pre C.1820 Longcase clock. The Victorians loved later carving GIII clocks though, and so beware buying one of these examples. Later carving would dramatically effect the clocks value.
  • Conclusion

    I wish you luck in your search. Buying from a recognized antique clock dealer may be slightly more expensive but you will get peace of mind. You should aim to get a money back guarantee that your clock is a genuine antique and a fully working example. It is easy to spend your good money buying a clock with a chequered history. Please take your time and not rush into a decision you will later regret.

    As they say an antique clock is for life and not just for Christmas. In our business we say, hard to find easy to sell. Original examples will provide you will years of pleasure and a good solid investment, Pendulum of Mayfair only sells top quality examples.