When the changing flux of a magnetic field crosses a conducting loop, there is a voltage induced in that loop. The voltage depends on the rate of change of that flux. In digital circuits, the wave energy moving down a transmission line involves a changing current. This changing current results in a changing H field. To calculate the induced voltage, the B field measure must be used. If the induction B field is in units of henries, then the coupled flux is BA, where A is loop area coupled to the flux. The induced voltage is therefore
where A is the coupling area in meters squared. The degree of coupling depends on circuit geometry and how rapidly the fields are changing. We will discuss parallel trace spacing and trace routing, as it relates to cross coupling, in later chapters.