Jim Kelley wrote:
Refer to a table of sines and observe the two things which vary
throughout the period of any sinusoidal wave. One of them is amplitude.
What would you prefer I call the other one?

Did you not understand what Gene Fuller said?
Io*cos(kx) is the amplitude term. If kx=0 then
the amplitude is Io. If kx=pi/4, the amplitude
is 0.707*Io. If kx=pi/2, the amplitude is zero.
cos(wt) does not vary with (x), only with time.
At any snapshot in time, e.g. t=0, the phase
does not vary at all.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com