If you check his website you will see that he is just expressing himself in an odd way and that you are missunderstanding his actual point.
When he says per cubic metre he actually means 'n', which means number of free electrons. This 'n' should use the same lenght unit as used elsewhere in the formula, so if you use m^3, you use m^-3, if you use ft^3, you use ft^-3.
I'm not sure if there is a previously established number for 'n', but Lyndon sets this as exactly 1 per cubic metre.
On his webpage, he says that as a formula this should be expressed as H = 2nhr/m. Note that the 2 in there is probably a typo on his part, but I copied and pasted the formula directly from his webpage.
This message has been edited by Melchior, 03-21-2005 12:34 PM