Page:Electricity (1912) Kapp.djvu/35

Rh the transmission of magnetic forces than air or brass or wood, and the degree to which this transmission is facilitated is called the magnetic permeability. In physical and engineering calculation the permeability (which is nothing else than the reciprocal of the ethereal coefficient) is indicated by the Greek symbol $$\mu$$, so that for magnetic forces the general equation for action over a distance $$D$$ becomes

$$F = \frac{1}{\mu} \frac{Mm}{D^2}$$

The suitability of any particular kind of iron for use in electrical machinery depends on the value of $$\mu$$, and the exact determination of this ethereal coefficient thus becomes a matter of practical importance. In making such determinations advantage is taken of certain relations which exist between electricity in the flowing state, commonly called electric currents, and magnetic effects. Since any flowing current represents energy, that is to say, is a dynamic phenomenon, such experiments have an electrodynamic character, and the unit of magnetic matter as defined above, under the arbitrary assumption that the magnetic ethereal coefficient for air is