ABSTRACT
This chapter explains most unreliable of instruments, the magnetic compass and takes as its case study the Arctic voyage of the Isabella and Alexander under the command of Captain John Ross. The global phenomenon of magnetic variation the angular difference between magnetic and true north had been known for centuries, as had the facts that it changed over time and varied as one moved across the globe. According to Captain John Ross, the voyage's impressive list of instruments included seven different types of magnetic compass, together with instruments such as dipping needles, for investigating the Earth's magnetic field. The Insulating Compass was designed by Henry Constantine Jennings, an idiosyncratic and irascible inventor of a rich miscellany of schemes. Jennings first contacted the Admiralty with a method of making a Compass in such a manner as always to point Due North, without any Variation in 1815.
