Early Electronic Television
Bell & Howell AFC Circuit
A voltage is taken from the audio discriminator, amplified, and fed to a panel meter. The pointer of the meter has been removed and replaced with a thin sheet of metal. The glass face of the meter has been replaced with a piece of plastic which has two thin pieces of metal attached.
As the voltage to the meter increases, the metal attached to the needle movement moves so as to come closer to the metal pieces on the plastic cover. The pieces of metal serve make a capacitor, which is part of the local oscillator tuning circuit. In this way the discriminator voltage changes the capacitance, and adjusts the local oscillator frequency to center it on the TV channel.
I think this arrangement was needed because the set has pushbutton motorized tuning, which is not accurate enough to tune the channels without some form of AFC.