The use of permanent magnets is a low cost method of establishing field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.
Electromagnetes in the form of an adjustable horseshoe magnet ( called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnetes only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.
Another way of indirectly inducing a magnetic field in a material is using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductor. Typically, one or more cylindrical component are hung from a solid copper bar running through the inside diameter. current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field within the test components.