The bell known as Big Ben weighs 13 tons and is the biggest of the five bells in the Elizabeth Tower. Big Ben only sounds at the top of every hour, and at that time it rings once for every hour (for example, it rings three times at 3 o'clock). The other four bells in the tower are smaller and play a short melody every 15 minutes. This melody, which is broadcasted live on BBC Radio 4 at 6 pm and midnight every day, can be heard in many other clocks around the world and is called the Westminster Chimes.