What had Faber designed that allowed him to be in constant contact with Montag?
Faber has invented a type of radio that looks like the Seashell Radio that Mildred and many others keep in their ears.
When Montag goes to Faber's house, seeking help to understand what it is that books have to offer, Faber reluctantly agrees, but he gives Montag a device which fits in the ear. Using this device Montag can hear Faber as if he were in a room with him. Having lived alone for years, Faber has toyed with electronics until he has succeeded in creating a transmitter and a receiver so that he can be in two-way contact with another person. He displays his listening device to Montag:
He [Faber] picked up a small green metal object no larger than a .22 bullet.
Faber tells Montag that he can sit in his home and listen in on the firemen:
"Eventually, I could put out ears into all parts of the city, with various men, listening and evaluating."
Faber apologizes for being cowardly as he sits in the safety of his home, but he agrees to transmit advice and encouragement to Montag if the fireman will keep the "bullet" in his ear. So, as the narrative develops Montag keeps this bullet in his ear and Faber advises him when Montag mistakenly thinks the friends of Millie will appreciate his reading of "Dover Beach." He tells Montag to throw the book away. Faber also talks to Montag when in the company of Beatty, but, unfortunately, Beatty discovers it.