This chapter discusses basic magnetism and measurement of electrical quantities. In order to quickly determine the direction of the magnetic field around a current-carrying conductor, the screw rule may be applied. There are major areas in which the magnetic effects of electric current are used. These are measuring instruments and motors, solenoids, electromagnets, inductors and transformers, etc. In large generators the magnetic field is provided by electromagnets rather than permanent magnets. The single loop is replaced by many such loops held in slots in an iron core. Inductors, or chokes as they are more popularly called, are used in many areas of modern technology. In electrical installation work the main application is in discharge lighting, where the choke is open-circuited across the ends of the tube to cause it to strike. The double-wound transformer is the commonest form of transformer and has a wide range of applications.