Pendulum

The regulator remained unfinished at the time of Harrison’s death. When later discovered, the regulator was fitted with a simple mahogany pendulum. This, however, was hardly what Harrison would have chosen for his regulator, since he had developed his ideas for a gridiron pendulum and employed them in his earlier clocks.

 

I trust anyone viewing these posts will be familiar with the working principle of a gridiron pendulum, therefore I shall restrict my account to the manufacture only.

 

The effectiveness of the gridiron pendulum is dependent on the different expansion ratios of the brass and steel used. The ratio of expansion [RER] between the steel and brass is the key factor in planning the pendulum. For this reason, I set about designing a test procedure to accurately measure expansion ratios relative to temperature changes.

 

Objective

To measure and record the change in length of brass and steel in relation to temperature.

The brass and steel used in these tests are from the same batch as that used for building the pendulum.

 

Method

The crude but effective apparatus for heating and measurement was to encase a 39-inch length of brass or steel in a copper construction filled with boiling water. Readings of length and temperature were logged as the water cooled to room temperature.

Readings of temperature were taken simultaneously by switching between five thermocouples attached to the sample.

I am indebted to my good friend John Spatcher, qualified in statistics and electronics, who was instrumental in the building of this test rig.

 

Results

I attach a graph of the test results, which surprised us both. The obvious linearity indicated an accurate and reliable basis to calculate expansion ratios.

With this data I enlisted the generous help of Peter Hastings who used the data to produce the correct pinning lengths for the assembly of the gridiron.

 

The large swing amplitude of the pendulum, up to 13 degrees, required care over aerodynamic and torsional stability in order to avoid fishtailing movement of the pendulum. 

The mass of the bob must be balanced in all planes, and this precluded the making of the bob in a traditional way.

I machined an aerodynamic 45-degree bevelled edge on the external diameter of a cast brass ring.  A brass sheet face was pressed into the front and an aluminium tongue fitted through the centreline. Molten lead was then poured into the central section of the bob around an aluminium tongue. The rear was then faced, and the tongue removed ready for assembly with a tradition screw adjustment.

Pendulum length and rate adjustment are made by a conventional compound adjusting screw under the bob.

 

Testing for temperature stability

 Harrison was aware of the importance of temperature for the stability of his clocks, and managed effective testing procedures even before the establishment of Fahrenheit and Centrigrade.

With the aid of my friend John Spatcher, a contactless measuring rig was devised whereby the light from a laser was interrupted by a flag attached to the base of the pendulum. The up and down movement could be recorded by a change in the strength of the laser beam, measured by a light sensor. This movement was then calibrated by micrometer adjustment to the suspension of the pendulum.

The expansion properties of the steel column, acting as a suspension point, were known and could be compared with the pendulum data indicated by the laser. Regret no photographic record made

 

Temperature changes were facilitated in Harrison’s preferred manner by leaving a workshop door open overnight to enable temperature recording down to 6 deg C and up to 16 deg C,

a much greater range than the clock would need to accommodate in practice.

 

Adjustment using the tin whistle pinning system, facilitated the adjustment for stability of the pendulum well within measurable expectation.

Pendulum assembly
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Bob with compound adjusment screw
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Wooden pattern
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Molten lead filling
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Rig for metal expansion testing
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Dial guage reading expansion
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Reading were plotted on a graph
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