In this video, when A wanted to talk to B, a microphone was connected to a distant speaker using two wires (A → B).
When B talked to A, a copy of above was used in the other direction (B → A) using two more wires.
This required four wires:
But then it says the two-way communication could be made by using just two wires:
How was this possible? Wouldn't the current from the microphone affect the speaker on the same side?
Best Answer
The modern telephone is wired in a Wheatstone bridge arrangement like this: -
So, if you ensure that the telephone network line impedance (\$Z_{LINE}\$) is controlled then, theoretically, any signal produced by the microphone is dramatically reduced into the local earpiece. It won't be a perfect cancellation but it'll be pretty good.
Amended picture originally from here. It might be easier to understand this diagram: -
Picture from here.
And, all throughout the network there are line amplifiers that need to translate from 2 wire to 4 wires so that amplifier circuits can be added: -
Picture from here. Then another hybrid transformer is used to reconvert the 2-way (4-wire) amplified signals back 2-wire: -
Here's an example of an early telephone anti-sidetone circuit using the same principle as the hybrid transformer: -
Picture from this website.