Whats the difference between an open circuit and a closed circuit in a magnet application?
A magnet operating by itself is referred to as an open circuit application. There are a lot of open circuit applications, like permanent magnets used to actuate Hall effect devices and reed switches. Helmholtz coil measurements are open circuit tests since no other magnetic materials are in the flux path during the test. The permeance coefficient of an open circuit magnet is determined by the magnet’s geometry alone. For example, a magnet with a length equal to its radius will have a PC close to 1.0. A true closed magnetic circuit would have the poles joined by a high permeability material and a Bd/Hd ratio of infinity, since Hd=0. This condition is approximated when a steel magnetizing fixture is used, or when magnets are tested in a permeameter. A true closed magnetic circuit has little practical value as no external flux is available to perform a function. However, many functional magnetic circuits are more closed than open, i.e. they have a high PC value. An estimate of the PC valu
