Sci/Hist: The lost explorers

This one is for teenagers and adults who like puns.

There is an old physics joke about an American who had a valuable Tait brand compass that he took to Australia, but in the outback, he found that his compass jammed. In the end, some local children rescued him and took him back to a cattle station and safety. He complained to a stockman that his compass had jammed, and the stockman asked to see the instrument. “Ah,” he said. “You brought a Tait’s compass with you. We have a saying here, that he who has a Tait’s is lost.”

There is a serious point here: the Earth’s magnetic field isn’t level, so a perfectly balanced and unmagnetised compass needle dips once it has been magnetised. The earth’s magnetic field dips away from the horizontal as you move away from the equator and this pulls one end downwards. The needle has to be filed and lightened at the lower end to balance this pull.

The problem is that the dip in the north is reversed in the south, so a compass made in England or America is lightened at the wrong end and will dip twice over, both from the magnetic force, and also from gravity. That means there is every chance that an English or North American compass will jam against the glass in the southern hemisphere, and give a false reading.

Richard Cunningham was never seen again after he wandered away from Major Mitchell’s party near the headwaters of the Bogan River in 1835, possibly because his compass jammed. This may also have happened to 18-year-old Henry Bryan in South Australia when he was separated from Governor Gawler’s party in 1839, not far from the Murray River.

Henry was carrying an English compass, and he was probably dehydrated. If he had not been delirious, he might have understood the problem, and tilted his compass to get an accurate reading. Whatever happened, Bryan’s body was never found.

A 19^th century dip circle.

Your challenge: find out what a dip circle is, and make one. A good one looks like the picture above, but William Gilbert, the first scientist to study magnets had one, and so did Edmond Halley (of Halley’s Comet fame). Theirs must have been simpler…

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