Strings

Although you'll find strings inside the piano, we don't call it a stringed instrument but a keyboard.

A finger touches a key
The key touches the action
The action sets the hammer into motion
The head of the hammer hits the string
The string is vibrating

The vibrating string touches the bridge
The bridge moves the soundboard
The sound reaches our ear

Strings spinmachine

If all the strings would have the same thickness, the speaking length should be twice as long with each octave we go down.
If the highest A is 5cm long, the lowest A should be 640 cm long.
An instrument this size would not fit into our homes. That's why we choose a shorter length and add to the thickness of the string.
The bass strings are wound with copper to get the sufficient mass we need for the right sound.

The division between length and thickness of the strings is called mensuration.