Big Ben

Many people think the giant clock attached to Westminster palace is called Big Ben but that was actually the name they gave to large 13 tonne bell inside the clock tower. The clock tower itself is called the Elizabeth tower. 

Not only does the Elizabeth clock tower have a giant bell but 4 smaller bells that play the 16 notes before the clock chimes. There are actually 5 bells and the big bell is called Big Ben. 

It's ok to call the giant clock Big Ben though. We all know what you mean. 

Source : Parliament Website