A primitive
device for controlling the return rate of a
phone dial. The
dial is
spring-loaded, so it spins back. However, you want it to return at a fixed rate, so it will transmit the
dial pulses at that rate.
The centrifugal friction device is attached to the dial and spins along with it. It has 2 small arms; spinning lifts them due to centrifugal force. This brings tiny brake pads at the tips of the arms into contact with a ring, and slows down the rotation.
If you take apart a rotary dial telephone, you may find one of these inside. If you take that apart, you won't be able to put it back together...