What is a Magnetic Compass, its principle, types and errors?

What are Adjustments and Repairs of Magnetic Compass/Who can make adjustments in magnetic compass?

In the UK, all adjustments should be made by a compass adjuster who holds a Certificate of Competency as Compass Adjuster issued by the UK Government.
 If a qualified compass adjuster is unavailable and the Master considers it necessary then adjustments may be made by a person holding a Certificate of Competency (Deck Officer), Class 1 (Master Mariner). The compass must be re-adjusted by a qualified compass adjuster at the next available opportunity. The date of any adjustment and other details should be noted in the compass deviation book.
The position of correctors should be recorded in the compass book and on deviation cards. Because the distances from the coefficients B and C correctors to the standard compass card and to the transmitting element are different, a transmitting magnetic compass will be overcompensated resulting in an error, which can be as much as 2½° and cannot be corrected. Separate deviation cards should be prepared for the standard compass and the transmitting magnetic compass repeater by comparing headings. Repairs should only be made by a compass manufacturer or other competent person using the proper test facilities.
When the work is finished the repairer should supply the owner or Master with a certificate, specifying that the work has been carried out in accordance with the necessary requirements ISO for Class A Compass and ISO 10316 for Class B Compass which are the International Standards for the Magnetic Compass.