Experiments are described in which the phenomenon of wave-interaction (‘Luxembourg effect’) is used to provide information about the height at which radio waves of different frequencies are absorbed in the ionosphere. It is first, demonstrated by two crucial experiments that the absorption mechanism suggested by Bailey & Martyn (1934
a
and
b
) is the true one. Measurements of the phase of the modulation transferred from one wave to the other by the non-linear absorption process in the ionosphere are described; and it is shown how, by measuring this phase at different modulation frequencies, it is possible to locate the region where the interaction occurs. The results of a series of experiments summarized in tables 2, 3
a
and 3
b
and figures 8 and 9 are discussed. The conclusion is reached that the frequency with which electrons collide with neutral molecules at a height of about 85 km. is of the order 5 x 10
5
sec.
-1
, and that this is the height near which the main absorption of waves of frequency 1 Mcyc./sec. and 200 kcyc./sec. are absorbed at night. Waves of frequency 90 and 68 kcyc./sec. are absorbed, and possibly also reflected, below this level. With the approach of dawn the regions responsible for absorbing 1 Mcyc./sec. and 200 kcyc./sec. waves drift apart. The theory of Bailey & Martyn (1934
b
) and Bailey (1937
a
) is related to modern theories of ionospheric absorption and is restated with the standard nomenclature of Appleton’s magneto-ionic theory.
A study of the interaction of radio waves