A mobile phone is in fact a small radio.A radio sends a person’s voice over a long way to another radio.A voice that is sent by radio is called a signal.A radio signal (信号) travels very quickly.
Only a few years ago,mobile phones were very large.They need very large batteries (电池).They had to be powerful to send their signal to far away places.This was because most cities had only one antenna tower for mobile phones.
Today’s mobile phones are small and easy to use.Now most cities have a lot of antenna towers,not just one.This means that each mobile phone doesn’t have to send its signal far away,so they don’t need to be so powerful.Mobile phones today use small batteries.A large city,where lots of mobile phones are used,can have hundreds of towers.