Mr Tompion's grande sonnerie longcase clock

If you have visited the Time and Society gallery at the ROG recently you may have noticed a glorious, yet empty, 17th century parquetry longcase. The clock movement and dial were removed for routine conservation and further study of its curious and complex movement.
D4100_1.jpgLongcase clock (ZAA0524)
Made by the 'father of English watchmaking', Thomas Tompion, the grande sonnerie movement runs for eight days. The term 'grande sonnerie' tells us that the clock strikes every quarter hour and follows each quarter strike by the previous hour. This particular clock counts the quarters on a high bell and the hours on a low bell. For example at half past 12 the clock will sound two high notes followed by 12 low notes.
Grande sonnerie clocks are rare, they were expensive to buy and far more demanding to make than regular hour striking clocks. In a 24 hour period a regular hour-striking clock will perform 156 hammer blows whereas this clock performs 768 blows in the same period.
D4100_3.jpg
Its ingenious design allows grande sonnerie striking using a single train and the same number of wheels as found in an hour-striking longcase clock. As far as we know this design of clock is unique. The quarter and hour striking is governed by a countwheel and cam system; one full turn of the countwheel equates to 384 blows of the hammer (12 hours striking) and each division counts the total number of blows required for the quarter; for example 11:15 requires 12 blows, 11:30 requires 13 blows and so on.
The method by which the hammers are activated or deactivated is dictated by a cam mounted in tandem to the countwheel. The cam follower is pivoted between the plates and has an arm that extends upwards through the movement terminating in a polished V section. This piece was humorously referred to as the 'snotty nose' by the late Dan Parkes, who restored the movement in the early 1950s. It was so-called because each hammer tail is combined with a small detent (finger) that 'wipes' with the V (nose) which flicks back and forth led by the cam follower and in doing so lifts the hammer tail away from the pin wheel. At any given time one of the hammers will always be disengaged from the striking train in this manner. The hammer tails are pivoted and sprung within a small frame (illustrated below) integral to the hammer arbor. This system has a very light action and requires minimal power from the train to engage and disengage the hammers though as can be seen from the dissembled arbor it would have been very labour intensive to construct and is a great example of Tompion's extraordinary skill in metal working.
tomp2.jpgtomp3.jpgHammer tails
The quality of construction is top class throughout and even internal parts that can only be seen when the clock is in pieces, such as the maintaining power bolt, are beautifully decorated and finished.
tomp1.jpgMaintaining power bolt
This clock is part of the R.K. Foulkes bequest and joined the collection in 1986.