When Montag goes to Faber and asks for his help, he asks specifically for help dealing with Captain Beatty.
"Can you help me in any way tonight, with the Fire Captain? I need an umbrella to keep off the rain. I'm so damned afraid I'll drown if he gets me again." (pg 89)
Faber finally admits to Montag that he has fiddled with electronics and radio transmissions. He feels that he is a terrible coward and his cowardice forced him to design the "green bullet". It is actually an ear plug that is no larger than a .22 bullet. He has never used it. He has never spoken to anyone on it for half his lifetime.
"If you put it in your ear, Montag, I can sit comfortably home, warming my frightened bones, and hear and analyze the firemen's world, find its weaknesses, without danger. I'm the Queen Bee, save in the hive. You will be the drone, the traveling ear." (pg 90)
He goes on to explain that if Montag should be caught, he would still be safe at home with
" a maximum of comfort and a minimum of chance." (pg 91)
Montag puts the earplug in his ear, and Faber puts another one in his own ear, and they can hear and talk to one another.
The page numbers I have given you are for my edition of the book, but they should be close.
As was mentioned in the previous post, Faber agrees to help Montag attempt to overthrow the firemen structure, and gives him the green bullet before he leaves. The green bullet is a small two-way communication device that resembles the Seashell radios that members of the dystopian society wear in their ears. The tiny green-metal object resembles a .22 bullet, which is why it is called the green bullet. When Montag places the green bullet in his ear, he can communicate with Faber, who is listening on the other end at a safe distance. The green bullet allows Montag and Faber to secretly communicate. Unfortunately, Captain Beatty discovers the green bullet while he is having a conversation with Montag. Immediately after Beatty knocks the green bullet out of Montag's ear, Montag kills Beatty by shooting him with a flamethrower.