Technology in the service of time
Big Ben
Big Ben is the name of the hour bell of the Great Clock at the top of St Stephen's Tower of the Palace of Westminster. The bell was cast in 1858 at the Whitechapel Bell Foundry and remains the largest bell that this London foundry has ever made. It rang for the first time in May 1859
The clock 
It was the Astronomer Royal, George Airy who drew up the specification for the clock for the new parliament buildings when they were rebuilt after a fire. He stated that: "the first stroke of the hour bell should register the time, correct to within one second per day, and furthermore that it should telegraph its performance twice a day to Greenwich Observatory where a record would be kept". This seemed technically impossible when the plans were made and it was not until 1851 that a clockmaker could be found to respond to the challenge. The clock was designed by Edmund Beckett Denison and built by Messrs E.J. Dent & Co.
Timekeeping
Big Ben is an excellent timekeeper, which has rarely stopped. Thwaites & Reed are the clockmakers who regulate the clock today and they know that if they add an old English penny to the pendulum it gains two fifths of a second a day. In this way they allow for the fact that the pendulum expands and contracts as the temperature rises or falls. With a pendulum as large as that in the Great Clock it is impossible to adjust the length by turning a screw to raise or lower the bob as in a domestic clock.
Materials failure and testing
On the 5th August 1976 the Great Clock was almost destroyed as a result of metal fatigue. A shaft broke as the clock started to chime which led to the total destruction of the chiming mechanism as the heavy weight (over a tonne of metal) fell faster and faster. Clockmakers had to remake the chiming train from scratch which took almost one year to complete.
Following this disaster the National Physical Laboratory Big Ben designed and fitted a fail-safe brake to the chiming mechanism to prevent such an accident ever happening again.
To prevent any similar disaster, every year there is a thorough check on all the parts of the Great Clock and the bells. The engineers use the techniques of non-destructive testing which they might otherwise use to inspect chemical plants and pipelines. For example they choose ultrasonic testing to look for internal flaws which might affect the hammers which strike the bells.
Submitted by: Andrew Hunt, 17 January 2007
