David Clark company's Voice Powered Communication System is an electrical version of two tin cans connected by a string. When you speak your voice travels to a transmitter causing fluctuations in a magnetic circuit. This creates a small amount of current (approx 20 micro amperes), which is enough to operate the receivers on up to six headsets.
The symmetrical design allows the headset to be worn on either side of the head. Recommended for noise levels to 100dB.