where a is a constant and A(x) has a value at position x from the beginning of the
arc that depends upon the proximity of that part of the arc to the electrolyte’s surface,
the amount of water on that surface and the extent that thermalisation has taken place
within the arc. To a first approximation, A can be considered to have an effective
value – i.e. a weighted average – that applies for the whole length of the arc