Magnet , around for ferrite and SmCo, about for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material. An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near (and especially inside) the coil, and its field lines are very similar to those of a magnet. The orientation of this effective magnet is determined by the right hand rule. The magnetic moment and the magnetic field of
Helmholtz coil coil is "n" and the current through the coils is "I", then the magnetic field B at the midpoint between the coils will be given by where formula_15 is the permeability of free space . Start with the formula for the on-axis field due to a single wire loop which is itself derived from the Biot–Savart law: Here The Helmholtz coils consists of "n" turns of wire, so the equivalent current in a one-turn coil is "n" times the current "I" in the "n"-turn coil. Substituting "nI" for "I" in the above formula gives the field for an "n"-turn coil: