"The compass is a ‘Jennings Insulating Compass’, which was said to be immune to magnetic deviation.

Only a little is known about its inventor, Henry Constantine Jennings, a manufacturing and consulting chemist said to be the first man to liquefy prussic acid, the inventor of a new way of killing moths and instigator of a campaign against the waste of stationery in the House of Commons. By the 1810s, he was certainly offering his inventions to the Admiralty, including his insulating compass, which he also submitted to the Board of Longitude in 1817, saying that it could be used for finding a ship’s longitude through the determination of magnetic variation.

In Jennings’ 1818 patent for the compass, he claims that he can protect the needle ‘from all action arising from iron in its neighbourhood’ by adding curved pieces of specially prepared iron to the bottom of the card. These, he says, ‘act as guards against the passage of the magnetic fluid, by absorbing the first quantity of it’, with specially treated iron filings in the compass body providing a second layer of magnetic ‘insulation’. The Admiralty and the Board of Longitude were not, however, convinced by Jennings’ claims, although some trials were finally agreed to.

These proved inconclusive but an insulating compass was also used by Captain John Ross during his 1818 Arctic expedition. Ross wrote that, ‘This instrument certainly answered the purpose for which it was intended, and completely obviated the effect of local attraction’. Others also seem to have been impressed, with Jennings claiming the support of no less than ‘2711 nautical men’, while Captain James Horsburgh, Hydrographer to the East India Company, recorded that Admiral Penrose had ‘tried it against a large magnet which would lift forty-two pounds of iron by its attractive power, but it did not influence the compass materially’.

Sadly, Jennings did little to ingratiate himself with Admiralty officials or the Board of Longitude, largely due to his aggressive style of letter-writing. Of the many wonderfully bad-tempered quotes, here’s just a couple. In 1818, he wrote, ‘it requires only Common Sense to judge of the case; & I am sorry the Board of Admiralty have proved themselves so deficient in that necessary quality’. And after the Board of Longitude rejected his application he expressed his sorrow that the Board ‘did not condescend to receive a Lesson on this Branch of Experimental Philosophy; which I should have been proud to have given them’. It is no surprise that internal correspondence referred to that ‘impudent Fellow Jennings’, a view that persisted when Jennings offered his compass again in the 1840s (when what is now the Museum’s example was tested and found to be easily deflected by iron) and then the 1860s, by which time his ideas were seen as nonsense. It would seem that Mr Jennings was perfectly insulated from the ability to influence others.”

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