In optical language, the wave sent out by the aerial would be called a plane polarised wave, the plane of polarisation being parallel to the magnetic force.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
Since only crystals possessing the property of double refraction show polarisation, it follows that those of the 1st, or cubic system-in which the diamond stands a prominent example-fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails at every point to transmit light, which a double-refracting gem allows to pass except when its optical axis is placed in the axis of the polariscope, but this will be dealt with more fully when the methods of testing the stones come to be considered.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin
The reason may have been a very simple one: the polarisation of signals returning from the Moon varies from one moment to the other, so if you have been transmitting with horizontal polarisation and go over to reception it is very easy to miss the answer of the other station if the polarisation has changed.
"The Dawn of Amateur Radio in the U.K. and Greece A Personal View"
Norman F. Joly