For Glinda has a big book in which is recorded every event that takes place anywhere in the world, just the moment that it happens, and so of course the book would tell her about the wireless message.
And that was the way Dorothy heard that the Historian wanted to speak with her, and there was a Shaggy Man in the Land of Oz who knew how to telegraph a wireless reply.

The result was that the Historian begged so hard to be told the latest news of Oz, so that he could write it down for the children to read, that Dorothy asked permission of Ozma and Ozma graciously consented.
That is why, after two long years of waiting, another Oz story is now presented to the children of America.

This would not have been possible had not some clever man invented the "wireless" and an equally clever child suggested the idea of reaching the mysterious Land of Oz by its means.
L.Frank Baum.
"OZCOT" at Hollywood in California LIST OF CHAPTERS 1 - Ojo and Unc Nunkie 2 - The Crooked Magician 3 - The Patchwork Girl 4 - The Glass Cat 5 - A Terrible Accident 6 - The Journey 7 - The Troublesome Phonograph 8 - The Foolish Owl and the Wise Donkey 9 - They Meet the Woozy 10 - Shaggy Man to the Rescue 11 - A Good Friend 12 - The Giant Porcupine 13 - Scraps and the Scarecrow 14 - Ojo Breaks the Law 15 - Ozma's Prisoner 16 - Princess Dorothy 17 - Ozma and Her Friends 18 - Ojo is Forgiven 19 - Trouble with the Tottenhots 20 - The Captive Yoop 21 - Hip Hopper the Champion 22 - The Joking Horners 23 - Peace is Declared 24 - Ojo Finds the Dark Well 25 - They Bribe the Lazy Quadling 26 - The Trick River 27 - The Tin Woodman Objects 28 - The Wonderful Wizard of Oz The Patchwork Girl of Oz.