s magnetic component current J, flowing through the loop. along the X axis i.e. the electric vector is at right angles to the magnetic vector, and both are perpendicular to the direction of propagation. The velocity is given by Equation 6.12. 6.1.2 Maxwell's equations and wave equations (6.12) Maxwell combined these laws and added the concept of displace- ment current to give a set of equations. These are expressed below in differential form, using the rationalised M.K.S. units as in Equa- If the relative permittivity and permeability are both unity, V is the tions 6.1 to 6.4. velocity of the wave in free space c, and a is the free space wavelength 2g The value of M is defined as (4) Henry per VD = p (6.1) metre, and measurements have set E at (8.8547)10-12 Farad per metre. This leads to a velocity of the wave in free space of (6.2) (2.998)108 metre per second. This is the same as the velocity determined by direct measurement, at least within the limits of VAE - - (6.3) experimental error. For most practical purposes it is adequate to take this velocity to be equal to 3 X 108 metres per second. The refractive index of a medium is defined by Equation 6.13. (6.4) (6.13) For an infinite homogeneous medium, with no free charges p and zero conductivity o, these equations become as in Equations 6.5 to By analogy to Ohm's law, the impedance of the medium is given 6.8, where E is the permittivity (dielectric constant) which equals by Equation 6.14. E E o' the relative permittivity multiplied by the permittivity of free space, and u is the magnetic permeability which equals M/ the relative permeability multiplied by the permeability of free space. (6.14) EVE = 0 (6.5) For free space this impedance is equal to 377 ohms. MVH=0 (6.6) The energy which is carried by the wave is given by the Poynting vector as EAH Watts per square metre. Historical understanding of VAE - - (6.7) the fields from electric charges and magnets led to the prediction of the electromagnetic wave travelling through space. Two major categories of receiv