On the electronic theory this is explained by stating that there is an accumulation of Electrons in one and of co-Electrons in the other.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
Inside the rods we have a movement of Electrons and co-Electrons to and fro, electric charges at the ends of the rods alternating with electric currents in the rods, the charges being at a maximum when the current is zero, and the current at a maximum when the charges have for the moment disappeared.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
The lines of strain in the external medium cannot contract or collapse quickly enough to keep up with the course of events, or movements of the Electrons in the rods, and hence their regular contraction and absorption is changed into a process of a different kind.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming