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Galbraith’s Account of the Foucault Pendulum Experiment in Dublin

In 1851, Léon Foucault amazed the world by demonstrating the rotation of the earth using a simple pendulum. As the earth spins, the swing-plane of a pendulum turns around. Within a month or so, the experiment was repeated in Dublin by two Irish scientists, Joseph Galbraith and Samuel Haughton, both fellows of Trinity College and both members of the Royal Irish Academy.

Engraving in L’Illustration of Foucault’s pendulum in the Panthéon, Paris
Engraving in L’Illustration of Foucault’s pendulum in the Panthéon, Paris

Galbraith kept a diary, which is now in the Manuscripts and Archives Research Library (TCD MS 3826). The entries for the months April to July, 1851 give us a day-by-day account of the activities of Galbraith and Haughton. The first relevant entry is for 17 April, recording that the two scientists were in Ringsend with Wilfred Haughton, Samuel’s cousin. Wilfred was Chief Engineer of the Dublin & Kingstown Railway, and the engine factory beside Grand Canal Basin, with its lofty roof, was an ideal location for the experiments. The pendulum length was 35.4 feet or 10.8 metres.

Following preliminary testing, six experiments were carried out, each lasting between 15 and 30 hours. The azimuthal angle of the pendulum, that is, the angle between the swing-plane and a north-south line, was recorded every 20 minutes, requiring one of the team members to be present throughout each experiment. The precession of the pendulum occurs slowly, taking well over a day to complete a full circle.

TCD MS 3826 diary entry for 17 April 1851
TCD MS 3826 diary entry for 17 April 1851

There are about 25 diary entries relevant to the experiments. They detail who was present during various periods. In the final experiment, a full rotation was achieved in a time of 28 hours and 26 minutes. The theoretical period is 28 hours and 21 minutes, not far from the observed period. According to an article in the Philosophical Magazine, “Messrs Galbraith and Haughton have pursued their research with all imaginable precautions”. Their impressive results confirm this assessment.

A full account of the experiments appears in the Proceedings of the Royal Irish Academy, Volume 116C, pages 1-15. Appendix A of this report contains a list of all the relevant entries from Galbraith’s diary. A copy of the report is available online.

Peter Lynch, School of Mathematics and Statistics, UCD

References

  • Lynch, Peter, 2016: Replication of Foucault’s pendulum experiment in Dublin. Proc. Roy. Irish Acad., 116C, 1-15. doi:10.3318/PRIAC.2016.116.03. (PDF: http://mathsci.ucd.ie/~plynch/Publications/PRIAC.pdf)
  • Manuscripts and Archives Research Library, Trinity College Dublin, TCD MS 3826, Diary of Joseph Galbraith for April and May 1851.