How signal is transmitted through RCA cable

[MarkWW]

How signal is transmitted through only one wire (red) of simple RCA cable?
What is use and how works ground wire (black)?

 

[sophiecentaur]

There are TWO conductors; the inner (red) and the outer braid. Together, they give you a circuit.

 

[MarkWW]

Cable transmit signal(sound) without black wire, I tested it.

 

[Baluncore]

Cable transmit signal(sound) without black wire, I tested it.

That is probably because there was another ground to ground connection present.
It may have been the other stereo channel, or through common earthing.

 

[sophiecentaur]

In your picture there is no "black wire' visible so it may be that there's a connection via the screen. The most likely explanation is that the case of the connector is connected to the Ground / case / Earth of the source of audio and the connector at the other end is connected to the ground at the amplifier (or whatever). So there is an 'unofficial' path joining the signal grounds at both ends together and making up the circuit. You may hear a difference in the sound quality with and without the black wire connected. Without a connection, there will almost certainly be some pickup of hum etc from the long path (possibly via the mains earth.
There ain't any magic at work here.
Edit: @Baluncore: Snap.

 

[MarkWW]

Thanks to everyone.