[4 11(a)], |dB| = 0 There is an interesting  relation between e0, the permittivity of free
space; m0, the permeability of free space; and c, the speed of light in vacuum:
(
)
0
0
0
0
4
µ4
ε µ
ε
 
 
=
π
 
 
 
π 
 
(
)
7
9
1
10
9
10
−
 
 
=  
 
 
 
×
8 2
2
1
1
(3
10 )
c
=
=
×
We will discuss this connection further in Chapter 8 on the
electromagnetic waves Since the speed of light in vacuum is constant,
the product m0e0 is fixed in magnitude