118Fundamental Electrical and Electronic Principles Fig. 4.8 rd(a)(b) Worked Example 4.5 Q A current of 400 mA is passed through a 550 turn coil wound on a toroid of mean diameter 8 cm. Calculate the magnetic fi eld strength. A I 0.4 A; N 550; d 8 10 2 m dHN metremampere turn/metre800 255500 40211...I225875 351so At/m H.Ans 4.7 Permeability of Free Space ( 0 ) We have seen in electrostatics that the permittivity of the dielectric is a measure of the ‘ willingness ’ of the dielectric to allow an electric ﬁ eld to exist in it. In magnetic circuits the corresponding quantity is the permeability of the material. If the magnetic ﬁ eld exists in a vacuum, then the ratio of the ﬂ ux density to the magnetic ﬁ eld strength is a constant, called the permeability of free space. 0henry/metreBH(4.4)compare this to farad/metreThe value forhen00εDE410 7rry/metre0 is used as the reference or datum level from which the permea-bilities of all other materials are measured.