A typographical error was made in Equation 2.

The specific resistance was expressed as:

$$ {\hbox{R }} = {\hbox{ Q}}/\Delta {{\hbox{P}}^{{2}}} $$

The specific resistance should be expressed as:

$$ {\hbox{R }} = \Delta {{\hbox{P}}^{{0.{5}}}}/{\hbox{Q}} $$