"The "vibratory circuit breaker" as Bell called it, was a long. light strip hanging from a pivot. When the receiver reed vibrated, it knocked the lower end of the strip to one side and kept hitting it before it could fall back again. While the strip remained in that position, a conductive crosspiece on it closed the local circuit, opening the circuit again when the reed stopped vibrating and allowed the strip to hang straight down. The battery-powered local circuit operated a Morse receiver."
Source: Alexender Graham Bell and the Conquest of Solitude by Robert V. Bruce,, pg. 136