Sound-powered phones were developed during World War II, and once they are wired up, they are powered solely by the sound of operator’s voice. Where a normal phone modulates an existing electrical current, these phones have a special microphone that creates the electrical current needed to communicate. Inside the mouthpiece, there is a transducer, which transforms the vibrations of the microphone into an electric current, which becomes sound on the receiver’s end. Unlike normal phones, these operate on a direct line, rather than connecting to a network of phones. The line is always active, and an operator only needs to lift the receiver to communicate. On some models there is a separate ring circuit, which is powered by electricity, to alert the operator on the other end. However, even without such a circuit, these phones are incredibly useful and durable. Sound-powered phones are more private and secure, for not being connected to a network, and also remain operational during a power failure. That makes them commonly used as emergency phones.