It’s a training film from 1961 that covers the sound-powered, local battery, and common battery systems along with the six basic components they use: generators, ringers, transmitters, receivers, induction coils, and capacitors. An induction coil increases the capabilities of this system, but capacitors are required to filter out the frequencies that would overload the receiver, passing only the higher speech frequencies. There are many advantages of a common battery system with regard to call switching: no local battery is necessary, nor is a generator needed at each station.