There are lots of areas of research and development where magnetic fields are needed, as well as in industrial measurement and testing technology. As a general rule, coils of different types and designs with fixed inductance are used for these tasks. 
The HUBERT amplifier family is the “driving” force for building up a magnetic field. It supplies the necessary coil current. Before operating with alternating current, it is helpful to clarify some of the relationships between current and voltage on this amplifier load. This is the topic of the application note.