The Count finds a telegraph office and convinces (with money) the telegraph dude to send a made-up telegraph.
Lucien Debray (Mrs. Danglars's lover) visits her to tell her that he has confidential information that there is about to be a revolution in Spain. He discovered this news via a particular telegraph report (ahem) that he got wind of (being the powerful government agent that he is).
Lucien Debray tells Mrs. Danglars to sell all of her Spanish bonds. She does.
The world is in uproar with the news of revolution in Spain; that is, until newspapers report that the news is false – there actually is no revolution in Spain.