How does a sound-powered phone transmit signals without electricity?

The principle behind a sound-powered phone relies on the conversion of sound waves into electrical signals without the need for an external power source. This is achieved through the use of a piezoelectric effect, where certain materials generate electricity when mechanical stress is applied. In a sound-powered phone, sound waves from the user's voice cause a small crystal or similar material within the device to vibrate. This vibration produces an electrical signal that can be transmitted to the other phone in the system.

The other choices suggest alternative methods of signal transmission that are not applicable in this context. For instance, batteries would be contrary to the design philosophy of sound-powered phones, which operate independently of external power. Similarly, while thermocouples convert heat into electricity, they do not play a role in the sound-powered phone’s functionality. Finally, although the analogy of cans connected by a string might illustrate basic sound transmission, it does not describe the specific mechanism employed by sound-powered phones to convert sound into an electrical signal.