Rotary dial in the Stasi Jail

The old Stasi Jail in Berlin was an amazing and awful place to visit. Each of the many interview rooms had one of these old-fashioned 1970s telephones. Younger viewers will never have used one. If you want to dial 1, you put your finger in that hole and turn it clockwise. When you hit the barrier in the bottom right, springs pull the dial back round. I never knew how it works but I've just looked it up. So, as the dial springs back it interrupts the current that flows. It does this a specific number of times for each digit: presumably most times for 0 because it will take longer to spring back. Each interruption produces a pulse of current. Each number has a different number of pulses which are decoded by the exchange to work out the number.