compensating magnet

It is fluid damped and incorporates a thermal expansion chamber and compensating magnet.

Adjustable compensating magnets are fitted and these are used to take out the effect of deviation when the compass is checked ( "swung" ).

This was improved again when Lord Kelvin patented in the 1880s another system of compass and which incorporated two compensating magnets.

However, because the magnetic "signature" of every ship changes slowly with location, and with time, it is necessary to adjust the compensating magnets, periodically, to keep the deviation errors to a practical minimum.