Unifilar variation compas and magnetometer, c 1840.

The balance point of a magnetic needle was of crucial importance when measuring the difference between geographic north (around which the earth spins) and magnetic north. This example, made of bras tipped with platinum, comes from a magnetometer made by Henry Barrow of London, in about 1840. The instrument was used by Sir John Richardson and William Pullen on the Arctic Expeditions searching for Sir John Franklin.