Natural Philosophy Collection

Hopkin’s Fork

For five years before David Thomson was appointed Professor of Natural Philosophy at King’s College, he had been acting Professor in the University of Glasgow. Among his charges in 1840 was the precocious William Thomson, who was justly to earn his reputation as one of the leading physical scientists of the century. In later life as Lord Kelvin, Thomson acknowledged the inspiration given him by his mentor. David Thomson never published any textbooks, but on his favourite subject of ‘acoustics’ he contributed a lengthy article to the Encyclopaedia Britannica. His account of some 25,000 words was one of the many scientific subjects treated in the Encyclopaedia at an astonishingly professional level for work aimed at the general public. Other articles, by contributors of the highest calibre including William Thomson himself, would in places tax the ability of the most able Honours students of today. In his account, David Thomson describes the now obscure acoustic demonstration piece known as Hopkin’s Fork, named by coincidence after William Thomson’s former mathematical coach at Peterhouse, Cambridge. The device is simply a bifurcated tube, designed to show that the mathematical predictions of how plates and membranes vibrate when sounded are true in practice. For example, if the prongs of the tube are placed over two areas of a vibrating plate that are predicted to vibrate out of phase, then a little sound reaches the single opening at the top of the tube. Our specimen is beautifully crafted, in the best tradition of the musical instrument maker’s art. John S Reid

John S. Reid