The drawing shows the principle of how a calorimeter functions. A particle enters from the left and hits a plate of lead or iron. The particle will then interact with the atoms of the plate so that secondary particles are created. These can be detected in a track chamber behind the plate.
This process continues through many layers of lead/iron plates and wire chambers until a little shower of secondary particles have been created. The energy of the incoming particle can be calculated by measurements of the activity inside the wire chambers.