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Magnetic dip circle

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made in London

Magnetic dip circle
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This instrument is a portable dip-circle for measuring the inclination of the Earth's magnetic field. It was made by Henry Barrow (1790-1870), a specialist instrument maker, who had been partner and then successor to Thomas Robinson, whose precision balances had used low-friction agate bearings, as do dip-cirlces.

The magnetic needle of this instrument, mounted on agate bearings (and thus very sensitive) moves in a vertical plane, measuring the variation in the Earth's magnetic field which is different over time. The needle is enclosed in a glazed wooden box, which can rotate on a lower graduated circle. Its circular scale is set outside the glazed box, and is set with microscopes so that very precise readings can be taken. It is engraved below the silvered scale: 'Henry Barrow & Co. 26 Oxendon St. London.' and 'No 33'.

Twelve dip-circles of this type were made by Henry Barrow for the Kew Observatory in about 1855. This particular instrument, Barrow's No. 33, was adopted as the Observatory standard instrument from 1860 until the magnetic observations ceased there in 1924.

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