In part two of "Fahrenheit 451", what homemade communication tool did Faber give Montag?

In Part Two of “Fahrenheit 451” called “The Sieve and the Sand”, Faber gives Montag a homemade device that they will be able to use to communicate with one another.  This device is a two-way radio that Montag will put into his ears so that he and Faber will be in continuous communication.  This two-way radio is used throughout the novel for the two men to have easy, private, and safe communication with one another.  It is not difficult for the men to hide the fact that they are communicating through these since other characters use them as a way to listen to their televisions and hear constant sound – Milred uses them in the novel.